code: func Meta(meta interface{}) CallOption {
  	return callOptionFunc(func(r *Request) error {
  		return r.SetMeta(meta)
  	})
  }
docs: // Meta returns a call option which attaches the given meta object to
  // the JSON-RPC 2.0 request (this is a Sourcegraph extension to JSON
  // RPC 2.0 for carrying metadata).