code: func (h *Client) ProductCustomMetric(productCustomMetric int64) *Client {
  	h.productCustomMetric = productCustomMetric
  	h.productCustomMetricSet = true
  	return h
  }
docs: // A product-level custom metric where metric index is a positive
  // integer between 1 and 200, inclusive. Product index must
  // be a positive integer between 1 and 200, inclusive. For
  // analytics.js the Enhanced Ecommerce plugin must be installed
  // before using this field.