code: func dial(req *http.Request, transport http.RoundTripper) (net.Conn, error) {
  	conn, err := DialURL(req.Context(), req.URL, transport)
  	if err != nil {
  		return nil, fmt.Errorf("error dialing backend: %v", err)
  	}
  
  	if err = req.Write(conn); err != nil {
  		conn.Close()
  		return nil, fmt.Errorf("error sending request: %v", err)
  	}
  
  	return conn, err
  }
docs: // dial dials the backend at req.URL and writes req to it.