Add .Elapsed in context.ResponseWriter for monitor purpose

With this commit we can record per requests's elapsed time,
so we can easy to monitor that by use a filter.

context/context.go

@@ -201,6 +201,7 @@ type Response struct {
 	http.ResponseWriter
 	Started bool
 	Status  int
+	Elapsed time.Duration
 }
 
 func (r *Response) reset(rw http.ResponseWriter) {
@@ -259,4 +260,4 @@ func (r *Response) Pusher() (pusher http.Pusher) {
 		return pusher
 	}
 	return nil
-}
+}