Add ActivityChange event

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.Current.net46.cs

@@ -1,13 +1,14 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.Tracing;
 using System.Threading;
 
 namespace System.Diagnostics
 {
     public partial class Activity
     {
-        private static readonly AsyncLocal<Activity?> s_current = new AsyncLocal<Activity?>();
+        private static readonly AsyncLocal<Activity?> s_current = new AsyncLocal<Activity?>(OnActivityChange);
 
         /// <summary>
         /// Gets or sets the current operation (Activity) for the current thread.  This flows
@@ -29,5 +30,13 @@ private static void SetCurrent(Activity? activity)
         {
             s_current.Value = activity;
         }
+
+        private static void OnActivityChange(AsyncLocalValueChangedArgs<Activity?> args)
+        {
+            if (DiagnosticSourceEventSource.Log.IsEnabled(EventLevel.Verbose, DiagnosticSourceEventSource.Keywords.ActivityChange))
+            {
+                DiagnosticSourceEventSource.Log.ActivityChange(args.CurrentValue?._spanId);
+            }
+        }
     }
 }

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticSourceEventSource.cs

@@ -186,6 +186,12 @@ public static class Keywords
             /// </summary>
             public const EventKeywords Events = (EventKeywords)0x2;
 
+            /// <summary>
+            /// Logs every time Activity.Current changes on any thread.
+            /// This allows to correlate any other event (like a CPU sample or an AllocationTick) to the activity it belongs to
+            /// </summary>
+            public const EventKeywords ActivityChange = (EventKeywords)0x4;
+
             // Some ETW logic does not support passing arguments to the EventProvider. To get around
             // this in common cases, we define some keywords that basically stand in for particular common arguments
             // That way at least the common cases can be used by everyone (and it also compresses things).
@@ -369,6 +375,12 @@ private void ActivityStart(string SourceName, string ActivityName, IEnumerable<K
         private void ActivityStop(string SourceName, string ActivityName, IEnumerable<KeyValuePair<string, string?>> Arguments) =>
             WriteEvent(12, SourceName, ActivityName, Arguments);
 
+        [Event(13, Keywords = Keywords.ActivityChange, Level = EventLevel.Verbose)]
+        public void ActivityChange(string? SpanId)
+        {
+            WriteEvent(13, SpanId);
+        }
+
         #region private
 
         private DiagnosticSourceEventSource()