Logging optimization.2 14 (#944)

* Trivial change to avoid an allocation when logging is off (normal case).

src/Microsoft.AspNetCore.Hosting/Internal/HostingApplication.cs

@@ -75,7 +75,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             return new Context
             {
                 HttpContext = httpContext,
-                Scope = scope,
+                Scope = scope,   // Scope can be null if logging is not on.   
                 StartTimestamp = startTimestamp,
             };
         }

src/Microsoft.AspNetCore.Hosting/Internal/HostingLoggerExtensions.cs

@@ -15,7 +15,11 @@ namespace Microsoft.AspNetCore.Hosting.Internal
     {
         public static IDisposable RequestScope(this ILogger logger, HttpContext httpContext)
         {
-            return logger.BeginScope(new HostingLogScope(httpContext));
+            // to avoid allocation, return a null scope if the logger is not on at least to some degree.   
+            if (logger.IsEnabled(LogLevel.Critical))
+                return logger.BeginScope(new HostingLogScope(httpContext));
+            else
+                return null;
         }
 
         public static void ApplicationError(this ILogger logger, Exception exception)