code: func newMetricVec(desc *Desc, newMetric func(lvs ...string) Metric) *metricVec {
  	return &metricVec{
  		metricMap: &metricMap{
  			metrics:   map[uint64][]metricWithLabelValues{},
  			desc:      desc,
  			newMetric: newMetric,
  		},
  		hashAdd:     hashAdd,
  		hashAddByte: hashAddByte,
  	}
  }
docs: // newMetricVec returns an initialized metricVec.