Fix a potential race in iterating counters (#28112)

Author: sywhang

src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/CounterGroup.cs

@@ -224,7 +224,7 @@ private void OnTimer()
                 // above, since WritePayload callback can contain user code that can invoke EventSource constructor
                 // and lead to a deadlock. (See https://github.com/dotnet/runtime/issues/40190 for details)
 
-                foreach (var counter in _counters)
+                foreach (var counter in counters)
                 {
                     // NOTE: It is still possible for a race condition to occur here. An example is if the session
                     // that subscribed to these batch of counters was disabled and it was immediately enabled in