Add runtime counter for TP work items produced (#24618)

Author: sywhang

src/System.Private.CoreLib/src/System/Diagnostics/Eventing/RuntimeEventSource.cs

@@ -23,6 +23,7 @@ internal sealed class RuntimeEventSource : EventSource
         private PollingCounter? _threadPoolThreadCounter;
         private IncrementingPollingCounter? _monitorContentionCounter;
         private PollingCounter? _threadPoolQueueCounter;
+        private IncrementingPollingCounter? _completedItemsCounter;
 
         private const int EnabledPollingIntervalMilliseconds = 1000; // 1 second
 
@@ -54,6 +55,7 @@ protected override void OnEventCommand(EventCommandEventArgs command)
                 _threadPoolThreadCounter = _threadPoolThreadCounter ?? new PollingCounter("threadpool-thread-count", this, () => ThreadPool.ThreadCount) { DisplayName = "ThreadPool Thread Count" };
                 _monitorContentionCounter = _monitorContentionCounter ?? new IncrementingPollingCounter("monitor-lock-contention-count", this, () => Monitor.LockContentionCount) { DisplayName = "Monitor Lock Contention Count", DisplayRateTimeScale = new TimeSpan(0, 0, 1) }; 
                 _threadPoolQueueCounter = _threadPoolQueueCounter ?? new PollingCounter("threadpool-queue-length", this, () => ThreadPool.PendingWorkItemCount) { DisplayName = "ThreadPool Queue Length" };
+                _completedItemsCounter = _completedItemsCounter ?? new IncrementingPollingCounter("threadpool-completed-items-count", this, () => ThreadPool.CompletedWorkItemCount) { DisplayName = "ThreadPool Completed Work Item Count", DisplayRateTimeScale = new TimeSpan(0, 0, 1) };
             }
         }
     }