Don't create the OpenTelemetryTags until we know we are going to use them (e.g. the activity is not ignored

Author: andrewlock

tracer/src/Datadog.Trace/Activity/Handlers/ActivityHandlerCommon.cs

@@ -216,7 +216,7 @@ static Scope CreateScopeFromActivity(T activity, OpenTelemetryTags? tags, SpanCo
             {
                 var span = Tracer.Instance.StartSpan(
                     activity.OperationName,
-                    tags: tags,
+                    tags: tags ?? new OpenTelemetryTags(),
                     parent: parent,
                     startTime: activity.StartTimeUtc,
                     traceId: traceId,

tracer/src/Datadog.Trace/Activity/Handlers/DefaultActivityHandler.cs

@@ -22,7 +22,7 @@ public bool ShouldListenTo(string sourceName, string? version)
 
         public void ActivityStarted<T>(string sourceName, T activity)
             where T : IActivity
-            => ActivityHandlerCommon.ActivityStarted(sourceName, activity, tags: new OpenTelemetryTags(), out _);
+            => ActivityHandlerCommon.ActivityStarted(sourceName, activity, tags: null, out _);
 
         public void ActivityStopped<T>(string sourceName, T activity)
             where T : IActivity