feat(observability): introduce minimal tracing implementation (#4105)

## Summary
This PR introduces a new tracing mechanism in GAX that allows recording
traces using OpenTelemetry. It provides a way of recording spans and
attributes, following the existing `ApiTracer` class pattern with a few
tracing-specific additions. The implementation is meant to be extensible
to support other implementations.

## New Classes
- **`TraceManager`**: An interface for managing spans and attributes;
can be implemented by observability frameworks.
- **`OpenTelemetryTraceManager`**: An implementation of `TraceManager`
that uses the OpenTelemetry API.
- **`AppCentricTracer`**: An `ApiTracer` implementation that delegates
span management to a `TraceManager`.
- **`AppCentricTracerFactory`**: A factory for creating
`AppCentricTracer` instances.
- **`ApiTracerContext`**: A context object that carries information
(like `EndpointContext`'s server address property) used to infer common
attributes for all tracers.
- **`Span`**: A handle returned by `TraceManager` to manage the
lifecycle of a specific span (ending it, recording errors, or setting
attributes).

## Approach
### Connecting Tracer with Manager
The implementation aims to decouple `AppCentricTracer` from
`TraceManager`. When a tracer starts an operation or an attempt, it
requests a `Span` from the recorder. This handle allows the tracer to
update the span (e.g., adding attributes or recording errors) to keep
`AppCentricTracer` separated from specific recorder implementations
(like OpenTelemetry's `Span` object).

### Attribute Inference via `ApiTracerContext`
To provide a source of Span Attributes that are common to all
operations, we introduced `ApiTracerContext`. This context is passed to
`ApiTracerFactory` and contains information such as serverAddress
(provided by `EndpointContext`). It is operated by `ClientContext`.
Initially, only `serverAddress` is contained in this class and it's
meant to obtain the `server.address` attribute.
The class is ultimately operated by `AppCentricTracer` to extract the
necessary attributes.

### Integration Tests
A new integration test, `ITOtelTracing`, was added to the
`java-showcase` module:
- It validates that the expected spans (operation and attempt spans) are
recorded with the correct names, parent-child relationships, and
attributes (including `server.address` and `gcp.client.language`).


#### Note on java-bigtable downstream check
Since
[SkipTrailersTest](https://github.com/googleapis/java-bigtable/blob/49fe7692c55747714ada4296ff0f856b1109eba5/google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/stub/SkipTrailersTest.java#L97)
mocks the tracer factory, the `EndpointContext` call to
`apiTracerFactory.withContext()` returns a null factory, causing a null
pointer exception when building the client context.
We expect the test to be adjusted with this change with the next
release.

### Confirmation in Cloud Trace
<img width="1801" height="843" alt="image"
src="https://github.com/user-attachments/assets/dfccf278-7bfc-43fe-bf64-51cae21494ea"
/>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: diegomarquezp

gax-java/dependencies.properties

@@ -40,6 +40,7 @@ maven.com_google_api_grpc_grpc_google_common_protos=com.google.api.grpc:grpc-goo
 maven.com_google_auth_google_auth_library_oauth2_http=com.google.auth:google-auth-library-oauth2-http:1.42.1
 maven.com_google_auth_google_auth_library_credentials=com.google.auth:google-auth-library-credentials:1.42.1
 maven.io_opentelemetry_opentelemetry_api=io.opentelemetry:opentelemetry-api:1.47.0
+maven.io_opentelemetry_opentelemetry_context=io.opentelemetry:opentelemetry-context:1.47.0
 maven.io_opencensus_opencensus_api=io.opencensus:opencensus-api:0.31.1
 maven.io_opencensus_opencensus_contrib_grpc_metrics=io.opencensus:opencensus-contrib-grpc-metrics:0.31.1
 maven.io_opencensus_opencensus_contrib_http_util=io.opencensus:opencensus-contrib-http-util:0.31.1

gax-java/gax/BUILD.bazel

@@ -19,6 +19,7 @@ _COMPILE_DEPS = [
     "@com_google_errorprone_error_prone_annotations//jar",
     "@com_google_guava_guava//jar",
     "@io_opentelemetry_opentelemetry_api//jar",
+    "@io_opentelemetry_opentelemetry_context//jar",
     "@io_opencensus_opencensus_api//jar",
     "@io_opencensus_opencensus_contrib_http_util//jar",
     "@io_grpc_grpc_java//context:context",

gax-java/gax/pom.xml

@@ -73,6 +73,11 @@
       <artifactId>opentelemetry-api</artifactId>
       <optional>true</optional>
     </dependency>
+    <dependency>
+      <groupId>io.opentelemetry</groupId>
+      <artifactId>opentelemetry-context</artifactId>
+      <optional>true</optional>
+    </dependency>
     <!--    Logging dependency    -->
     <dependency>
       <groupId>org.slf4j</groupId>

gax-java/gax/src/main/java/com/google/api/gax/rpc/ClientContext.java

@@ -41,6 +41,7 @@
 import com.google.api.gax.core.ExecutorAsBackgroundResource;
 import com.google.api.gax.core.ExecutorProvider;
 import com.google.api.gax.rpc.internal.QuotaProjectIdHidingCredentials;
+import com.google.api.gax.tracing.ApiTracerContext;
 import com.google.api.gax.tracing.ApiTracerFactory;
 import com.google.api.gax.tracing.BaseApiTracerFactory;
 import com.google.auth.ApiKeyCredentials;
@@ -269,6 +270,11 @@ public static ClientContext create(StubSettings settings) throws IOException {
     if (watchdogProvider != null && watchdogProvider.shouldAutoClose()) {
       backgroundResources.add(watchdog);
     }
+    ApiTracerContext apiTracerContext =
+        ApiTracerContext.newBuilder()
+            .setServerAddress(endpointContext.resolvedServerAddress())
+            .build();
+    ApiTracerFactory apiTracerFactory = settings.getTracerFactory().withContext(apiTracerContext);
 
     return newBuilder()
         .setBackgroundResources(backgroundResources.build())
@@ -284,7 +290,7 @@ public static ClientContext create(StubSettings settings) throws IOException {
         .setQuotaProjectId(settings.getQuotaProjectId())
         .setStreamWatchdog(watchdog)
         .setStreamWatchdogCheckIntervalDuration(settings.getStreamWatchdogCheckIntervalDuration())
-        .setTracerFactory(settings.getTracerFactory())
+        .setTracerFactory(apiTracerFactory)
         .setEndpointContext(endpointContext)
         .build();
   }

gax-java/gax/src/main/java/com/google/api/gax/rpc/EndpointContext.java

@@ -40,6 +40,7 @@
 import com.google.auto.value.AutoValue;
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Strings;
+import com.google.common.net.HostAndPort;
 import java.io.IOException;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -133,6 +134,8 @@ public static EndpointContext getDefaultInstance() {
 
   public abstract String resolvedEndpoint();
 
+  public abstract String resolvedServerAddress();
+
   public abstract Builder toBuilder();
 
   public static Builder newBuilder() {
@@ -228,6 +231,8 @@ public abstract static class Builder {
 
     public abstract Builder setResolvedEndpoint(String resolvedEndpoint);
 
+    public abstract Builder setResolvedServerAddress(String serverAddress);
+
     public abstract Builder setResolvedUniverseDomain(String resolvedUniverseDomain);
 
     abstract Builder setUseS2A(boolean useS2A);
@@ -382,6 +387,23 @@ boolean shouldUseS2A() {
       return mtlsEndpoint().contains(Credentials.GOOGLE_DEFAULT_UNIVERSE);
     }
 
+    private String parseServerAddress(String endpoint) {
+      if (Strings.isNullOrEmpty(endpoint)) {
+        return endpoint;
+      }
+      String hostPort = endpoint;
+      if (hostPort.contains("://")) {
+        // Strip the scheme if present. HostAndPort doesn't support schemes.
+        hostPort = hostPort.substring(hostPort.indexOf("://") + 3);
+      }
+      try {
+        return HostAndPort.fromString(hostPort).getHost();
+      } catch (IllegalArgumentException e) {
+        // Fallback for cases HostAndPort can't handle.
+        return hostPort;
+      }
+    }
+
     // Default to port 443 for HTTPS. Using HTTP requires explicitly setting the endpoint
     private String buildEndpointTemplate(String serviceName, String resolvedUniverseDomain) {
       return serviceName + "." + resolvedUniverseDomain + ":443";
@@ -416,7 +438,9 @@ String mtlsEndpointResolver(
     public EndpointContext build() throws IOException {
       // The Universe Domain is used to resolve the Endpoint. It should be resolved first
       setResolvedUniverseDomain(determineUniverseDomain());
-      setResolvedEndpoint(determineEndpoint());
+      String endpoint = determineEndpoint();
+      setResolvedEndpoint(endpoint);
+      setResolvedServerAddress(parseServerAddress(endpoint));
       setUseS2A(shouldUseS2A());
       return autoBuild();
     }

gax-java/gax/src/main/java/com/google/api/gax/tracing/ApiTracerContext.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package com.google.api.gax.tracing;
+
+import com.google.api.core.InternalApi;
+import com.google.auto.value.AutoValue;
+import java.util.HashMap;
+import java.util.Map;
+import javax.annotation.Nullable;
+
+/**
+ * A context object that contains information used to infer attributes that are common for all
+ * {@link ApiTracer}s.
+ *
+ * <p>For internal use only.
+ */
+@InternalApi
+@AutoValue
+public abstract class ApiTracerContext {
+
+  /**
+   * @return a map of attributes to be included in attempt-level spans
+   */
+  public Map<String, String> getAttemptAttributes() {
+    Map<String, String> attributes = new HashMap<>();
+    if (getServerAddress() != null) {
+      attributes.put(AppCentricAttributes.SERVER_ADDRESS_ATTRIBUTE, getServerAddress());
+    }
+    return attributes;
+  }
+
+  @Nullable
+  public abstract String getServerAddress();
+
+  public static Builder newBuilder() {
+    return new AutoValue_ApiTracerContext.Builder();
+  }
+
+  @AutoValue.Builder
+  public abstract static class Builder {
+    public abstract Builder setServerAddress(String serverAddress);
+
+    public abstract ApiTracerContext build();
+  }
+}

gax-java/gax/src/main/java/com/google/api/gax/tracing/ApiTracerFactory.java

@@ -61,4 +61,15 @@ enum OperationType {
    * @param operationType the type of operation that the tracer will trace
    */
   ApiTracer newTracer(ApiTracer parent, SpanName spanName, OperationType operationType);
+
+  /**
+   * Returns a new {@link ApiTracerFactory} that will use the provided context to infer attributes
+   * for all tracers created by the factory.
+   *
+   * @param context an {@link ApiTracerContext} object containing information to construct
+   *     attributes
+   */
+  default ApiTracerFactory withContext(ApiTracerContext context) {
+    return this;
+  }
 }

gax-java/gax/src/main/java/com/google/api/gax/tracing/AppCentricAttributes.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package com.google.api.gax.tracing;
+
+import com.google.api.core.BetaApi;
+import com.google.api.core.InternalApi;
+
+/**
+ * Utility class with common attribute names in app-centric observability.
+ *
+ * <p>For internal use only.
+ */
+@InternalApi
+@BetaApi
+public class AppCentricAttributes {
+  /** The address of the server being called (e.g., "pubsub.googleapis.com"). */
+  public static final String SERVER_ADDRESS_ATTRIBUTE = "server.address";
+}

gax-java/gax/src/main/java/com/google/api/gax/tracing/AppCentricTracer.java

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package com.google.api.gax.tracing;
+
+import com.google.api.core.BetaApi;
+import com.google.api.core.InternalApi;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * An implementation of {@link ApiTracer} that uses a {@link TraceManager} to record traces. This
+ * implementation is agnostic to the specific {@link TraceManager} in order to allow extensions that
+ * interact with other backends.
+ */
+@BetaApi
+@InternalApi
+public class AppCentricTracer implements ApiTracer {
+  public static final String LANGUAGE_ATTRIBUTE = "gcp.client.language";
+
+  public static final String DEFAULT_LANGUAGE = "Java";
+
+  private final TraceManager traceManager;
+  private final Map<String, String> attemptAttributes;
+  private final String attemptSpanName;
+  private final ApiTracerContext apiTracerContext;
+  private TraceManager.Span attemptHandle;
+
+  /**
+   * Creates a new instance of {@code AppCentricTracer}.
+   *
+   * @param traceManager the {@link TraceManager} to use for recording spans
+   * @param attemptSpanName the name of the individual attempt spans
+   */
+  public AppCentricTracer(
+      TraceManager traceManager, ApiTracerContext apiTracerContext, String attemptSpanName) {
+    this.traceManager = traceManager;
+    this.attemptSpanName = attemptSpanName;
+    this.apiTracerContext = apiTracerContext;
+    this.attemptAttributes = new HashMap<>();
+    buildAttributes();
+  }
+
+  private void buildAttributes() {
+    this.attemptAttributes.put(LANGUAGE_ATTRIBUTE, DEFAULT_LANGUAGE);
+    this.attemptAttributes.putAll(this.apiTracerContext.getAttemptAttributes());
+  }
+
+  @Override
+  public void attemptStarted(Object request, int attemptNumber) {
+    Map<String, String> attemptAttributes = new HashMap<>(this.attemptAttributes);
+    // Start the specific attempt span with the operation span as parent
+    this.attemptHandle = traceManager.createSpan(attemptSpanName, attemptAttributes);
+  }
+
+  @Override
+  public void attemptSucceeded() {
+    endAttempt();
+  }
+
+  private void endAttempt() {
+    if (attemptHandle != null) {
+      attemptHandle.end();
+      attemptHandle = null;
+    }
+  }
+}