LiveMetrics background thread safeguards added to never throw unhandled exception

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Version 2.11.0-beta2
  - [Add NetStandard2.0 Target for WindowsServerPackage](https://github.com/Microsoft/ApplicationInsights-dotnet-server/issues/1212)
  - [Add NetStandard2.0 Target for DependencyCollector](https://github.com/Microsoft/ApplicationInsights-dotnet-server/issues/1212)
+ - [QuickPulse/LiveMetrics background thread safeguards added to never throw unhandled exception.](https://github.com/microsoft/ApplicationInsights-dotnet-server/issues/1088)
 
 ## Version 2.11.0-beta1
  - [Add support for Event Counter collection.](https://github.com/Microsoft/ApplicationInsights-dotnet-server/issues/1222)

Src/PerformanceCollector/Perf.Shared.NetFull/Implementation/QuickPulse/Helpers/QuickPulseThreadModuleScheduler.cs

@@ -2,6 +2,7 @@
 {
     using System;
     using System.Threading;
+    using Microsoft.ApplicationInsights.Extensibility.Implementation.Tracing;
 
     internal class QuickPulseThreadModuleScheduler : IQuickPulseModuleScheduler
     {
@@ -47,7 +48,23 @@ public void Dispose()
 
             private void Worker(object state)
             {
-                (state as Action<CancellationToken>)?.Invoke(this.cancellationTokenSource.Token);
+                try
+                {
+                    (state as Action<CancellationToken>)?.Invoke(this.cancellationTokenSource.Token);
+                }
+                catch (Exception ex)
+                {
+                    // This is a Thread, and we don't want any exception thrown ever from this part as this would cause application crash.
+                    try
+                    {
+                        QuickPulseEventSource.Log.UnknownErrorEvent(ex.ToInvariantString());
+                    }
+                    catch (Exception)
+                    {
+                        // Intentionally empty. If EventSource writing itself is failing as well, there is nothing more to be done here.
+                        // The best that can be done is atleast prevent application crash due to unhandledexception from Thread.
+                    }
+                }
             }
         }
     }