dbMetricsMonitorsMu sync.Mutex
	dbMetricsMonitors   map[string]struct{}
)

// startCompactorOnce start one compactor per transport. If the interval get smaller on repeated calls, the
// compactor is replaced. A destroy func is returned. If all destroy funcs with the same transport are called,
// the compactor is stopped.
func startCompactorOnce(c storagebackend.TransportConfig, interval time.Duration) (func(), error) {
	compactorsMu.Lock()