code: func (r *Request) SetMeta(v interface{}) error {
  	b, err := json.Marshal(v)
  	if err != nil {
  		return err
  	}
  	r.Meta = (*json.RawMessage)(&b)
  	return nil
  }
docs: // SetMeta sets r.Meta to the JSON representation of v. If JSON
  // marshaling fails, it returns an error.