Move Logging to Middleware Package (#1070)

* Use a specialized ResponseWriter in middleware

* Track User & Upstream in RequestScope

* Wrap responses in our custom ResponseWriter

* Add tests for logging middleware

* Inject upstream metadata into request scope

* Use custom ResponseWriter only in logging middleware

* Assume RequestScope is never nil

pkg/apis/middleware/scope.go

@@ -34,6 +34,9 @@ type RequestScope struct {
 	// SessionRevalidated indicates whether the session has been revalidated since
 	// it was loaded or not.
 	SessionRevalidated bool
+
+	// Upstream tracks which upstream was used for this request
+	Upstream string
 }
 
 // GetRequestScope returns the current request scope from the given request