// adding the given name prefix. The extraLabels are added to each metric as
// constant labels.
func (m *MetricValues) ToPromMetrics(namePrefix string, extraLabels map[string]string,
) []prometheus.Metric {
	metrics := make([]prometheus.Metric, 0, len(m.values))
	for metricName, mv := range m.values {
		desc := m.descriptors[metricName]
		promDesc := desc.toPromDesc(namePrefix, extraLabels)
		for _, v := range mv {