Feature for setting activity tags on creation

src/Hosting/Hosting/src/Internal/HostingApplicationDiagnostics.cs

@@ -388,7 +388,7 @@ private void RecordRequestStartMetrics(HttpContext httpContext)
     private Activity? StartActivity(HttpContext httpContext, bool loggingEnabled, bool diagnosticListenerActivityCreationEnabled, out bool hasDiagnosticListener)
     {
         hasDiagnosticListener = false;
-
+        var tagsForCreation = httpContext.Features.Get<IHttpActivityCreationTagsFeature>()?.ActivityCreationTags;
         var headers = httpContext.Request.Headers;
         var activity = ActivityCreator.CreateFromRemote(
             _activitySource,
@@ -402,7 +402,7 @@ private void RecordRequestStartMetrics(HttpContext httpContext)
             },
             ActivityName,
             ActivityKind.Server,
-            tags: null,
+            tags: tagsForCreation,
             links: null,
             loggingEnabled || diagnosticListenerActivityCreationEnabled);
         if (activity is null)

src/Http/Http.Features/src/IHttpActivityCreationTagsFeature.cs

@@ -0,0 +1,18 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.AspNetCore.Http.Features;
+
+/// <summary>
+/// Provides a mechanism to add tags to the <see cref="System.Diagnostics.Activity"/> at creation time for incoming HTTP requests.
+/// These tags can be used for tracing when making sampling decisions.
+/// </summary>
+public interface IHttpActivityCreationTagsFeature
+{
+    /// <summary>
+    /// A collection of tags to be added to the <see cref="System.Diagnostics.Activity"/> when it is created for the current HTTP request.
+    /// These tags are available at Activity creation time and can be used for sampling decisions.
+    /// </summary>
+    /// <returns>Tags to add to the Activity at creation time.</returns>
+    IEnumerable<KeyValuePair<string, object?>>? ActivityCreationTags { get; }
+}

src/Http/Http.Features/src/PublicAPI.Unshipped.txt

@@ -1 +1,3 @@
 #nullable enable
+Microsoft.AspNetCore.Http.Features.IHttpActivityCreationTagsFeature
+Microsoft.AspNetCore.Http.Features.IHttpActivityCreationTagsFeature.ActivityCreationTags.get -> System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string!, object?>>?