Replace non-threadsafe HashSet with ConcurrentDictionary in RequestTrackingTelemetryModule.IsHandlerToFilter

Src/Web/Web.Shared.Net/RequestTrackingTelemetryModule.cs

@@ -28,11 +28,9 @@ public class RequestTrackingTelemetryModule : ITelemetryModule
         private TelemetryConfiguration telemetryConfiguration;
         private bool initializationErrorReported;
         private ChildRequestTrackingSuppressionModule childRequestTrackingSuppressionModule = null;
-
-        /// <summary>
-        /// Handler types that are not TransferHandlers will be included in request tracking.
-        /// </summary>
-        private HashSet<Type> requestHandlerTypesDoNotFilter = new HashSet<Type>();
+
+        /// <summary>Tracks if given type should be included in telemetry. ConcurrentDictionary is used as a concurrent hashset.</summary>
+        private ConcurrentDictionary<Type, bool> includedHttpHandlerTypes = new ConcurrentDictionary<Type, bool>();
 
         /// <summary>
         /// Gets or sets a value indicating whether child request suppression is enabled or disabled. 
@@ -371,7 +369,7 @@ private bool IsHandlerToFilter(IHttpHandler handler)
             if (handler != null)
             {
                 var handlerType = handler.GetType();
-                if (!this.requestHandlerTypesDoNotFilter.Contains(handlerType))
+                if (!this.includedHttpHandlerTypes.ContainsKey(handlerType))
                 {
                     var handlerName = handlerType.FullName;
                     foreach (var h in this.Handlers)
@@ -383,7 +381,7 @@ private bool IsHandlerToFilter(IHttpHandler handler)
                         }
                     }
 
-                    this.requestHandlerTypesDoNotFilter.Add(handlerType);
+                    this.includedHttpHandlerTypes.TryAdd(handlerType, true);
                 }
             }