rules/manager.go
func (m *Manager) Collect(ch chan<- prometheus.Metric) {
	for _, g := range m.RuleGroups() {
		ch <- prometheus.MustNewConstMetric(groupInterval,
			prometheus.GaugeValue,
			g.interval.Seconds(),
			groupKey(g.file, g.name))
	}
}
func (m *Manager) Collect(ch chan<- prometheus.Metric) {
	for _, g := range m.RuleGroups() {
		ch <- prometheus.MustNewConstMetric(groupInterval,
			prometheus.GaugeValue,
			g.interval.Seconds(),
			groupKey(g.file, g.name))
	}
}
// Collect implements prometheus.Collector.