code: func (v *HistogramVec) WithLabelValues(lvs ...string) Observer {
  	h, err := v.GetMetricWithLabelValues(lvs...)
  	if err != nil {
  		panic(err)
  	}
  	return h
  }
docs: // WithLabelValues works as GetMetricWithLabelValues, but panics where
  // GetMetricWithLabelValues would have returned an error. Not returning an
  // error allows shortcuts like
  //     myVec.WithLabelValues("404", "GET").Observe(42.21)