code: func MustNewConstMetric(desc *Desc, valueType ValueType, value float64, labelValues ...string) Metric {
  	m, err := NewConstMetric(desc, valueType, value, labelValues...)
  	if err != nil {
  		panic(err)
  	}
  	return m
  }
docs: // MustNewConstMetric is a version of NewConstMetric that panics where
  // NewConstMetric would have returned an error.