code: func proxyStream(w http.ResponseWriter, r *http.Request, url *url.URL) {
  	// TODO(random-liu): Set MaxBytesPerSec to throttle the stream.
  	handler := proxy.NewUpgradeAwareHandler(url, nil /*transport*/, false /*wrapTransport*/, true /*upgradeRequired*/, &responder{})
  	handler.ServeHTTP(w, r)
  }
docs: // proxyStream proxies stream to url.