fix(aspnetcore): skip shutdown hook when ExceptionlessLifetimeService is registered

UseExceptionless() now checks if ExceptionlessLifetimeService is already
registered via AddExceptionless() hosting integration and skips the
redundant ApplicationStopping callback to avoid double queue flushing.

Author: niemyjski

src/Platforms/Exceptionless.AspNetCore/ExceptionlessExtensions.cs

@@ -49,8 +49,10 @@ public static IApplicationBuilder UseExceptionless(this IApplicationBuilder app,
                 new ExceptionlessDiagnosticListener(client),
                 eventName => ExceptionlessDiagnosticListener.IsRelevantEvent(eventName));
 
-            var lifetime = app.ApplicationServices.GetRequiredService<IHostApplicationLifetime>();
-            lifetime.ApplicationStopping.Register(() => client.ProcessQueueAsync().ConfigureAwait(false).GetAwaiter().GetResult());
+            if (app.ApplicationServices.GetService<Exceptionless.Extensions.Hosting.ExceptionlessLifetimeService>() == null) {
+                var lifetime = app.ApplicationServices.GetRequiredService<IHostApplicationLifetime>();
+                lifetime.ApplicationStopping.Register(() => client.ProcessQueueAsync().ConfigureAwait(false).GetAwaiter().GetResult());
+            }
 
             return app.UseMiddleware<ExceptionlessMiddleware>(client);
         }