code: func (e *Engine) PrometheusCollectors() []prometheus.Collector {
  	var metrics []prometheus.Collector
  	metrics = append(metrics, tsdb.PrometheusCollectors()...)
  	metrics = append(metrics, tsi1.PrometheusCollectors()...)
  	metrics = append(metrics, tsm1.PrometheusCollectors()...)
  	metrics = append(metrics, e.retentionEnforcer.PrometheusCollectors()...)
  	return metrics
  }
docs: // PrometheusCollectors returns all the prometheus collectors associated with
  // the engine and its components.