Avoid accessing Id on activity start

src/OpenTelemetry/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+* Prevent accessing activity Id before sampler runs in case of legacy
+  activities.
+  ([2659](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2659))
+
 * Added `ReadOnlyTagCollection` and expose `Tags` on `MetricPoint` instead of
   `Keys`+`Values`
   ([#2642](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2642))

src/OpenTelemetry/Internal/OpenTelemetrySdkEventSource.cs

@@ -105,7 +105,14 @@ public void ActivityStarted(Activity activity)
         {
             if (this.IsEnabled(EventLevel.Verbose, EventKeywords.All))
             {
-                this.ActivityStarted(activity.OperationName, activity.Id);
+                // Accessing activity.Id here will cause the Id to be initialized
+                // before the sampler runs in case where the activity is created using legacy way
+                // i.e. new Activity("Operation name"). This will result in Id not reflecting the
+                // correct sampling flags
+                // https://github.com/dotnet/runtime/issues/61857
+                var activityId = string.Concat("00-", activity.TraceId.ToHexString(), "-", activity.SpanId.ToHexString());
+                activityId = string.Concat(activityId, activity.ActivityTraceFlags.HasFlag(ActivityTraceFlags.Recorded) ? "-01" : "-00");
+                this.ActivityStarted(activity.OperationName, activityId);
             }
         }