code: func checkWait(resp *http.Response) (int, bool) {
  	switch r := resp.StatusCode; {
  	// any 500 error code and 429 can trigger a wait
  	case r == errors.StatusTooManyRequests, r >= 500:
  	default:
  		return 0, false
  	}
  	i, ok := retryAfterSeconds(resp)
  	return i, ok
  }
docs: // checkWait returns true along with a number of seconds if the server instructed us to wait
  // before retrying.