code: func checkHealth(serverURL *url.URL, backend *BackendConfig) error {
  	req, err := backend.newRequest(serverURL)
  	if err != nil {
  		return fmt.Errorf("failed to create HTTP request: %s", err)
  	}
  
  	req = backend.addHeadersAndHost(req)
  
  	client := http.Client{
  		Timeout:   backend.Options.Timeout,
  		Transport: backend.Options.Transport,
  	}
  
  	resp, err := client.Do(req)
  	if err != nil {
  		return fmt.Errorf("HTTP request failed: %s", err)
  	}
  
  	defer resp.Body.Close()
  
  	if resp.StatusCode < http.StatusOK || resp.StatusCode >= http.StatusBadRequest {
  		return fmt.Errorf("received error status code: %v", resp.StatusCode)
  	}
  
  	return nil
  }
docs: // checkHealth returns a nil error in case it was successful and otherwise
  // a non-nil error with a meaningful description why the health check failed.