code: 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))
  	}
  }
docs: // Collect implements prometheus.Collector.