Type           MetricType
	Help           string
	VariableLabels []string

	// managed values follow:
	labelSet map[string]struct{}
}

func (md *MetricDescriptor) getLabelSet() map[string]struct{} {
	if md.labelSet != nil {
		return md.labelSet
	}
	md.labelSet = make(map[string]struct{}, len(md.VariableLabels))
	for _, label := range md.VariableLabels {
		md.labelSet[label] = struct{}{}
	}