Reset counters within Timer thread

Author: eterekhin

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

@@ -17,6 +17,7 @@ internal sealed class CounterGroup
         private readonly EventSource _eventSource;
         private readonly List<DiagnosticCounter> _counters;
         private static readonly object s_counterGroupLock = new object();
+        private bool _resetCounters = true;
 
         internal CounterGroup(EventSource eventSource)
         {
@@ -155,7 +156,6 @@ private void EnableTimer(float pollingIntervalInSeconds)
             if (_pollingIntervalInMilliseconds == 0 || pollingIntervalInSeconds * 1000 < _pollingIntervalInMilliseconds)
             {
                 _pollingIntervalInMilliseconds = (int)(pollingIntervalInSeconds * 1000);
-                ResetCounters(); // Reset statistics for counters before we start the thread.
 
                 _timeStampSinceCollectionStarted = DateTime.UtcNow;
                 _nextPollingTimeStamp = DateTime.UtcNow + new TimeSpan(0, 0, (int)pollingIntervalInSeconds);
@@ -223,6 +223,12 @@ private void OnTimer()
                 DiagnosticCounter[] counters;
                 lock (s_counterGroupLock)
                 {
+                    if (_resetCounters)
+                    {
+                        ResetCounters();
+                        _resetCounters = false;
+                    }
+
                     now = DateTime.UtcNow;
                     elapsed = now - _timeStampSinceCollectionStarted;
                     pollingIntervalInMilliseconds = _pollingIntervalInMilliseconds;