code: func newHTTPSink(u string, timeout time.Duration, headers http.Header, transport *http.Transport, listeners ...httpStatusListener) *httpSink {
  	if transport == nil {
  		transport = http.DefaultTransport.(*http.Transport)
  	}
  	return &httpSink{
  		url:       u,
  		listeners: listeners,
  		client: &http.Client{
  			Transport: &headerRoundTripper{
  				Transport: transport,
  				headers:   headers,
  			},
  			Timeout: timeout,
  		},
  	}
  }
docs: // newHTTPSink returns an unreliable, single-flight http sink. Wrap in other
  // sinks for increased reliability.