Start adding a Logback demo for opencensus-contrib-log-correlation-stackdriver (Java).

java/log_correlation/stackdriver/logback/README.md

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+# OpenCensus/Logback/Stackdriver Log Correlation Demo
+
+An application that demonstrates log correlation in Stackdriver with
+`opencensus-contrib-log-correlation-stackdriver`. The application contains SLF4J log statements and
+OpenCensus tracing instrumentation. It configures logging with a Logback XML configuration and
+exports logs using the
+[Stackdriver Logging Logback appender](https://cloud.google.com/logging/docs/setup/java#logback_appender).
+It also exports traces using `opencensus-exporter-trace-stackdriver`, so that Stackdriver can show
+the log entries associated with each trace.
+
+The application requires a Google Cloud project with Stackdriver Logging enabled. It can be run with
+`./gradlew run`.

java/log_correlation/stackdriver/logback/build.gradle

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+description = 'OpenCensus/Logback/Stackdriver Log Correlation Demo'
+
+apply plugin: 'java'
+
+repositories {
+    mavenCentral()
+}
+
+group = "io.opencensus"
+version = "0.16.0-SNAPSHOT"
+
+def opencensusVersion = "0.15.0"
+
+tasks.withType(JavaCompile) {
+    sourceCompatibility = '1.7'
+    targetCompatibility = '1.7'
+}
+
+dependencies {
+    compile "io.opencensus:opencensus-api:${opencensusVersion}",
+            "io.opencensus:opencensus-exporter-trace-stackdriver:${opencensusVersion}",
+            "org.slf4j:slf4j-api:1.7.25"
+
+    runtime "io.opencensus:opencensus-contrib-log-correlation-stackdriver:${opencensusVersion}",
+            "io.opencensus:opencensus-impl-lite:${opencensusVersion}",
+            "com.google.cloud:google-cloud-logging-logback:0.48.0-alpha"
+}
+
+apply plugin: 'application'
+
+mainClassName =
+    'io.opencensus.demo.logcorrelation.stackdriver.logback.OpenCensusTraceLoggingEnhancerDemo'

java/log_correlation/stackdriver/logback/gradle/wrapper/gradle-wrapper.properties

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+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.9-bin.zip
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists

java/log_correlation/stackdriver/logback/src/main/java/io/opencensus/demo/logcorrelation/stackdriver/logback/OpenCensusTraceLoggingEnhancerDemo.java

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+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.demo.logcorrelation.stackdriver.logback;
+
+import io.opencensus.common.Scope;
+import io.opencensus.exporter.trace.stackdriver.StackdriverTraceConfiguration;
+import io.opencensus.exporter.trace.stackdriver.StackdriverTraceExporter;
+import io.opencensus.trace.Sampler;
+import io.opencensus.trace.Tracer;
+import io.opencensus.trace.Tracing;
+import io.opencensus.trace.samplers.Samplers;
+import java.io.IOException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/** Demo for {@code OpenCensusTraceLoggingEnhancer} using SLF4J and Logback. */
+public final class OpenCensusTraceLoggingEnhancerDemo {
+
+  private static final Logger logger =
+      LoggerFactory.getLogger(OpenCensusTraceLoggingEnhancerDemo.class.getName());
+
+  private static final Tracer tracer = Tracing.getTracer();
+
+  private OpenCensusTraceLoggingEnhancerDemo() {}
+
+  /**
+   * Runs the demo. It creates a sampled and a non-sampled trace to demonstrate how log correlation
+   * handles both cases.
+   */
+  public static void main(String[] args) throws IOException {
+    StackdriverTraceExporter.createAndRegister(StackdriverTraceConfiguration.builder().build());
+    createSampledTrace();
+    createNonSampledTrace();
+    Tracing.getExportComponent().shutdown();
+  }
+
+  private static void createSampledTrace() {
+    doWork(Samplers.alwaysSample());
+  }
+
+  private static void createNonSampledTrace() {
+    doWork(Samplers.neverSample());
+  }
+
+  private static void doWork(Sampler sampler) {
+    try (Scope scope = tracer.spanBuilder("ParentSpan").setSampler(sampler).startScopedSpan()) {
+      pause();
+      logger.warn("parent span log message 1");
+      pause();
+      doMoreWork();
+      pause();
+      logger.info("parent span log message 2");
+      pause();
+    }
+  }
+
+  private static void doMoreWork() {
+    try (Scope scope = tracer.spanBuilder("ChildSpan").startScopedSpan()) {
+      pause();
+      logger.info("child span log message 1");
+      pause();
+      logger.error("child span log message 2");
+      pause();
+    }
+  }
+
+  /** Sleeps for 500 ms to spread out the events on the trace. */
+  private static void pause() {
+    try {
+      Thread.sleep(500);
+    } catch (InterruptedException e) {
+      // ignore
+    }
+  }
+}