Log dropped telemetry item count on shutdown

src/OpenTelemetry/BatchExportProcessor.cs

@@ -186,6 +186,8 @@ protected override bool OnShutdown(int timeoutMilliseconds)
             this.shutdownDrainTarget = this.circularBuffer.AddedCount;
             this.shutdownTrigger.Set();
 
+            OpenTelemetrySdkEventSource.Log.DroppedExportProcessorItems(this.droppedCount, this);
+
             if (timeoutMilliseconds == Timeout.Infinite)
             {
                 this.exporterThread.Join();

src/OpenTelemetry/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+* `BatchExportProcessor.OnShutdown` will now log the count of dropped telemetry items.
+  ([#2331](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2331))
+
 ## 1.2.0-alpha3
 
 Released 2021-Sep-13

src/OpenTelemetry/Internal/OpenTelemetrySdkEventSource.cs

@@ -135,6 +135,22 @@ public void MissingPermissionsToReadEnvironmentVariable(SecurityException ex)
             }
         }
 
+        [NonEvent]
+        public void DroppedExportProcessorItems(long droppedCount, object exporter)
+        {
+            if (droppedCount <= 0 && this.IsEnabled(EventLevel.Informational, EventKeywords.All))
+            {
+                this.ZeroDroppedExportProcessorItems();
+                return;
+            }
+
+            // Positive number of dropped items
+            if (this.IsEnabled(EventLevel.Warning, EventKeywords.All))
+            {
+                this.ExistsDroppedExportProcessorItems(droppedCount, exporter.GetType());
+            }
+        }
+
         [Event(1, Message = "Span processor queue size reached maximum. Throttling spans.", Level = EventLevel.Warning)]
         public void SpanProcessorQueueIsExhausted()
         {
@@ -309,6 +325,18 @@ public void MissingPermissionsToReadEnvironmentVariable(string exception)
             this.WriteEvent(30, exception);
         }
 
+        [Event(31, Message = "Zero telemetry items were dropped.", Level = EventLevel.Informational)]
+        public void ZeroDroppedExportProcessorItems()
+        {
+            this.WriteEvent(31);
+        }
+
+        [Event(32, Message = "Telemetry items dropped: '{0}', type: '{1}'.", Level = EventLevel.Warning)]
+        public void ExistsDroppedExportProcessorItems(long droppedCount, Type type)
+        {
+            this.WriteEvent(32, droppedCount, type.ToString());
+        }
+
 #if DEBUG
         public class OpenTelemetryEventListener : EventListener
         {