code: func (h *Client) ProductImpressionCustomMetric(productImpressionCustomMetric int64) *Client {
  	h.productImpressionCustomMetric = productImpressionCustomMetric
  	h.productImpressionCustomMetricSet = true
  	return h
  }
docs: // A product-level custom metric where metric index is a positive
  // integer between 1 and 200, inclusive. Impression List index
  // must be 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.