code: func (h *HTTPTester) Once() (time.Duration, error) {
  	h.br.Seek(0, io.SeekStart)
  
  	start := time.Now()
  	resp, err := h.client.Post(h.url, "application/json", h.br)
  	if err != nil {
  		return 0, err
  	}
  	defer resp.Body.Close()
  
  	if resp.StatusCode != http.StatusOK {
  		return 0, fmt.Errorf("received %s", resp.Status)
  	}
  	dur := time.Since(start)
  
  	return dur, nil
  }
docs: // Once executes a single test request.