code: func (r *RoundTripper) Close() error {
  	r.mutex.Lock()
  	defer r.mutex.Unlock()
  	for _, client := range r.clients {
  		if err := client.Close(); err != nil {
  			return err
  		}
  	}
  	r.clients = nil
  	return nil
  }
docs: // Close closes the QUIC connections that this RoundTripper has used