satellite/metabase/segmentloop: add missing monitoring

Currently we are tracking loop time with RunOnce method
metrics but it may give inaccurate values if loop if waiting
for observer for a ver long time. We had such case with GC
which is only loop observer in GC peer and its joning loop
every 5 days.

Change-Id: I08546c912e00c3641488de6a5d75948fe75c8e99

satellite/metabase/segmentloop/service.go

@@ -352,6 +352,8 @@ func (loop *Service) Wait() {
 var errNoObservers = errs.New("no observers")
 
 func (loop *Service) iterateDatabase(ctx context.Context, observers []*observerContext) (err error) {
+	defer mon.Task()(&ctx)(&err)
+
 	defer func() {
 		if err != nil {
 			errorObservers(observers, err)