code: func (h *Client) CustomMetric(customMetric int64) *Client {
  	h.customMetric = customMetric
  	h.customMetricSet = true
  	return h
  }
docs: // Each custom metric has an associated index. There is a maximum
  // of 20 custom metrics (200 for Premium accounts). The metric
  // index must be a positive integer between 1 and 200, inclusive.