NLogTarget - Use FlushAsync instead of synchronous

Author: snakefoot

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## VNext
 - [Populate required field Message with "n/a" if it is empty](https://github.com/microsoft/ApplicationInsights-dotnet/issues/1066)
+- [NLog Target with support for specifying ConnectionString](https://github.com/microsoft/ApplicationInsights-dotnet/issues/2897)
 
 ## Version 2.22.0
 - no changes since beta.

LOGGING/src/NLogTarget/ApplicationInsightsTarget.cs

@@ -187,18 +187,7 @@ 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)
             {