code: func (h *Client) CustomDimension(customDimension string) *Client {
  	h.customDimension = customDimension
  	h.customDimensionSet = true
  	return h
  }
docs: // Each custom dimension has an associated index. There is
  // a maximum of 20 custom dimensions (200 for Premium accounts).
  // The dimension index must be a positive integer between 1
  // and 200, inclusive.