Extract StopActivitySlow as it's a rarely-called code path

Author: andrewlock

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

@@ -255,6 +255,16 @@ public static void ActivityStopped<T>(string sourceName, T activity)
                     }
                 }
 
+                StopActivitySlow(sourceName, activity);
+            }
+            catch (Exception ex)
+            {
+                Log.Error(ex, "Error processing the DefaultActivityHandler.ActivityStopped callback");
+            }
+
+            static void StopActivitySlow<TInner>(string sourceName, TInner activity)
+                where TInner : IActivity
+            {
                 // The listener didn't send us the Activity or the scope instance was not found
                 // In this case we are going go through the dictionary to check if we have an activity that
                 // has been closed and then close the associated scope.
@@ -319,10 +329,6 @@ public static void ActivityStopped<T>(string sourceName, T activity)
                     }
                 }
             }
-            catch (Exception ex)
-            {
-                Log.Error(ex, "Error processing the DefaultActivityHandler.ActivityStopped callback");
-            }
 
             static void CloseActivityScope<TInner>(string sourceName, TInner activity, Scope scope)
                 where TInner : IActivity