code: function(key, value) {
        if(!/(dimension|metric)[0-9]+/i.test(key)) {
          warn('custom: key must match dimension[0-9]+ or metric[0-9]+');
          return;
        }
  
        _set.call(this, 'set', key, value);
  
        return this;
      }
docs: https://developers.google.com/analytics/devguides/collection/analyticsjs/custom-dims-mets