code: func WithResponseWriter(ctx context.Context, w http.ResponseWriter) (context.Context, http.ResponseWriter) {
  	irw := instrumentedResponseWriter{
  		ResponseWriter: w,
  		Context:        ctx,
  	}
  	return &irw, &irw
  }
docs: // WithResponseWriter returns a new context and response writer that makes
  // interesting response statistics available within the context.