few mote usefull comments

discovery/manager.go

@@ -70,6 +70,8 @@ func NewManager(logger log.Logger) *Manager {
 }
 
 // Manager maintains a set of discovery providers and sends each update to a channel used by other packages.
+// Targets are grouped by target set names.
+// When a given target set doesn't include any targets the manager doesn't send any updates for this target set.
 type Manager struct {
 	logger         log.Logger
 	actionCh       chan func(context.Context)
@@ -181,6 +183,8 @@ func (m *Manager) allGroups() map[string][]*targetgroup.Group {
 		for pkey, tsets := range m.targets {
 			del := true
 			for _, tg := range tsets {
+				// Don't add a target set if the target group has no targets.
+				// This happens when a Discoverer sends an empty targets array for a group to indicate that these targets have been removed.
 				if len(tg.Targets) != 0 {
 					tSetsAll[pkey.setName] = append(tSetsAll[pkey.setName], tg)
 					del = false