code: func HTTPChecker(r string, statusCode int, timeout time.Duration, headers http.Header) health.Checker {
  	return health.CheckFunc(func() error {
  		client := http.Client{
  			Timeout: timeout,
  		}
  		req, err := http.NewRequest("HEAD", r, nil)
  		if err != nil {
  			return errors.New("error creating request: " + r)
  		}
  		for headerName, headerValues := range headers {
  			for _, headerValue := range headerValues {
  				req.Header.Add(headerName, headerValue)
  			}
  		}
  		response, err := client.Do(req)
  		if err != nil {
  			return errors.New("error while checking: " + r)
  		}
  		if response.StatusCode != statusCode {
  			return errors.New("downstream service returned unexpected status: " + strconv.Itoa(response.StatusCode))
  		}
  		return nil
  	})
  }
docs: // HTTPChecker does a HEAD request and verifies that the HTTP status code
  // returned matches statusCode.