Merge pull request #375 from benaadams/log-scope-only-enabled

Proposal: Start log scope only if logging is enabled

src/Microsoft.AspNet.Hosting/Internal/HostingEngine.cs

@@ -105,7 +105,9 @@ namespace Microsoft.AspNet.Hosting.Internal
 
                     try
                     {
-                        using (logger.BeginScope("Request Id: {RequestId}", requestIdentifier))
+                        using (logger.IsEnabled(LogLevel.Critical)
+                            ? logger.BeginScope("Request Id: {RequestId}", requestIdentifier) 
+                            : null)
                         {
                             await application(httpContext);
                         }