NLog ApplicationInsightsTarget FlushAsync (#3005)

Author: snakefoot

LOGGING/src/NLogTarget/ApplicationInsightsTarget.cs

@@ -27,7 +27,6 @@ public sealed class ApplicationInsightsTarget : TargetWithLayout
     {
         private TelemetryClient telemetryClient;
         private TelemetryConfiguration telemetryConfiguration;
-        private DateTime lastLogEventTime;
         private NLog.Layouts.Layout connectionStringLayout = string.Empty;
 
         /// <summary>
@@ -150,8 +149,6 @@ protected override void Write(LogEventInfo logEvent)
                 throw new ArgumentNullException(nameof(logEvent));
             }
 
-            this.lastLogEventTime = DateTime.UtcNow;
-
             if (logEvent.Exception != null)
             {
                 // TODO: Uncomment once  ExceptionTelemetry is tested and verified.
@@ -181,18 +178,7 @@ protected override void FlushAsync(AsyncContinuation asyncContinuation)
 
             try
             {
-                this.TelemetryClient.Flush();
-                if (DateTime.UtcNow.AddSeconds(-30) > this.lastLogEventTime)
-                {
-                    // Nothing has been written, so nothing to wait for
-                    asyncContinuation(null);
-                }
-                else
-                {
-                    // Documentation says it is important to wait after flush, else nothing will happen
-                    // https://docs.microsoft.com/azure/application-insights/app-insights-api-custom-events-metrics#flushing-data
-                    System.Threading.Tasks.Task.Delay(TimeSpan.FromMilliseconds(500)).ContinueWith((task) => asyncContinuation(null));
-                }
+                this.telemetryClient.FlushAsync(System.Threading.CancellationToken.None).ContinueWith(t => asyncContinuation(t.Exception));
             }
             catch (Exception ex)
             {