code: func GetResponseWriter(ctx context.Context) (http.ResponseWriter, error) {
  	v := ctx.Value("http.response")
  
  	rw, ok := v.(http.ResponseWriter)
  	if !ok || rw == nil {
  		return nil, ErrNoResponseWriterContext
  	}
  
  	return rw, nil
  }
docs: // GetResponseWriter returns the http.ResponseWriter from the provided
  // context. If not present, ErrNoResponseWriterContext is returned. The
  // returned instance provides instrumentation in the context.