[aspnet] Add url.path to sampler tags

src/OpenTelemetry.Instrumentation.AspNet.TelemetryHttpModule/ActivityHelper.cs

@@ -27,6 +27,9 @@ internal static class ActivityHelper
         TelemetryHttpModule.AspNetSourceName,
         typeof(ActivityHelper).Assembly.GetPackageVersion());
 
+    [ThreadStatic]
+    private static KeyValuePair<string, object?>[]? cachedTagsStorage;
+
     /// <summary>
     /// Try to get the started <see cref="Activity"/> for the running <see
     /// cref="HttpContext"/>.
@@ -60,7 +63,24 @@ public static bool HasStarted(HttpContext context, out Activity? aspNetActivity)
     {
         PropagationContext propagationContext = textMapPropagator.Extract(default, context.Request, HttpRequestHeaderValuesGetter);
 
-        Activity? activity = AspNetSource.StartActivity(TelemetryHttpModule.AspNetActivityName, ActivityKind.Server, propagationContext.ActivityContext);
+        KeyValuePair<string, object?>[]? tags;
+        if (context.Request?.Unvalidated?.Path is string path)
+        {
+            tags = cachedTagsStorage ??= new KeyValuePair<string, object?>[1];
+
+            tags[0] = new KeyValuePair<string, object?>("url.path", path);
+        }
+        else
+        {
+            tags = null;
+        }
+
+        Activity? activity = AspNetSource.StartActivity(TelemetryHttpModule.AspNetActivityName, ActivityKind.Server, propagationContext.ActivityContext, tags);
+
+        if (tags is not null)
+        {
+            tags[0] = default;
+        }
 
         if (activity != null)
         {

src/OpenTelemetry.Instrumentation.AspNet.TelemetryHttpModule/CHANGELOG.md

@@ -2,6 +2,14 @@
 
 ## Unreleased
 
+* `TelemetryHttpModule` will now pass the `url.path` tag (set to
+  [Request.Unvalidated.Path](https://learn.microsoft.com/dotnet/api/system.web.unvalidatedrequestvalues.path))
+  when starting `Activity` instances for incoming requests so that it is
+  available to samplers and may be used to influence the sampling decision made
+  by [custom
+  implementations](https://github.com/open-telemetry/opentelemetry-dotnet/tree/main/docs/trace/extending-the-sdk#sampler).
+  ([#1871](https://github.com/open-telemetry/opentelemetry-dotnet-contrib/pull/1871))
+
 ## 1.9.0-beta.1
 
 Released 2024-Jun-18

src/OpenTelemetry.Instrumentation.AspNet/Implementation/HttpInListener.cs

@@ -64,7 +64,6 @@ private void OnStartActivity(Activity activity, HttpContext context)
             }
 
             var request = context.Request;
-            var requestValues = request.Unvalidated;
 
             // see the spec https://github.com/open-telemetry/semantic-conventions/blob/v1.24.0/docs/http/http-spans.md
             var originalHttpMethod = request.HttpMethod;
@@ -83,8 +82,6 @@ private void OnStartActivity(Activity activity, HttpContext context)
                 activity.SetTag(SemanticConventions.AttributeNetworkProtocolVersion, protocolVersion);
             }
 
-            activity.SetTag(SemanticConventions.AttributeUrlPath, requestValues.Path);
-
             // TODO url.query should be sanitized
             var query = url.Query;
             if (!string.IsNullOrEmpty(query))

src/OpenTelemetry.Instrumentation.AspNet/README.md

@@ -137,6 +137,18 @@ below.
 
 ### Trace Filter
 
+> [!NOTE]
+> OpenTelemetry has the concept of a
+[Sampler](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#sampling).
+When using `OpenTelemetry.Instrumentation.AspNet.TelemetryHttpModule` the
+`url.path` tag is supplied automatically to samplers when telemetry is started
+for incoming requests. It is recommended to use a sampler which inspects
+`url.path` (as opposed to the `Filter` option described below) in order to
+perform filtering as it will prevent child spans from being created and bypass
+data collection for anything NOT recorded by the sampler. The sampler approach
+will reduce the impact on the process being instrumented for all filtered
+requests.
+
 This instrumentation by default collects all the incoming http requests. It
 allows filtering of requests by using the `Filter` function in
 `AspNetTraceInstrumentationOptions`. This defines the condition for allowable
@@ -158,11 +170,6 @@ this.tracerProvider = Sdk.CreateTracerProviderBuilder()
     .Build();
 ```
 
-It is important to note that this `Filter` option is specific to this
-instrumentation. OpenTelemetry has a concept of a
-[Sampler](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#sampling),
-and the `Filter` option does the filtering *before* the Sampler is invoked.
-
 ### Trace Enrich
 
 This instrumentation library provides `EnrichWithHttpRequest`,

test/OpenTelemetry.Instrumentation.AspNet.Tests/HttpInListenerTests.cs

@@ -142,6 +142,11 @@ public void AspNetRequestsAreCollectedSuccessfully(
                     options.RecordException = recordException;
                 })
                 .AddInMemoryExporter(exportedItems)
+                .SetSampler(
+                    new TestSampler(SamplingDecision.RecordAndSample)
+                    {
+                        ExpectedUrlPath = expectedUrlPath,
+                    })
                 .Build())
             {
                 using var inMemoryEventListener = new InMemoryEventListener(AspNetInstrumentationEventSource.Log);
@@ -307,8 +312,16 @@ private class TestSampler(SamplingDecision samplingDecision) : Sampler
     {
         private readonly SamplingDecision samplingDecision = samplingDecision;
 
+        public string? ExpectedUrlPath { get; set; }
+
         public override SamplingResult ShouldSample(in SamplingParameters samplingParameters)
         {
+            if (!string.IsNullOrEmpty(this.ExpectedUrlPath))
+            {
+                Assert.NotNull(samplingParameters.Tags);
+                Assert.Contains(samplingParameters.Tags, t => t.Key == "url.path" && (t.Value as string) == this.ExpectedUrlPath);
+            }
+
             return new SamplingResult(this.samplingDecision);
         }
     }