fix: Check collection released before target checks (#39841)

Related to #39840

The target could be updated async in previous code. This PR make remove
collection from target observer block until all tasks related in
dispatchers are removed preventing the metrics being updated after
collection released.

---------

Signed-off-by: Congqi Xia <[email protected]>

internal/querycoordv2/observers/target_observer.go

@@ -219,7 +219,9 @@ func (ob *TargetObserver) schedule(ctx context.Context) {
 				delete(ob.readyNotifiers, req.CollectionID)
 				ob.mut.Unlock()
 
+				ob.keylocks.Lock(req.CollectionID)
 				ob.targetMgr.RemoveCollection(ctx, req.CollectionID)
+				ob.keylocks.Unlock(req.CollectionID)
 				req.Notifier <- nil
 			case ReleasePartition:
 				ob.targetMgr.RemovePartition(ctx, req.CollectionID, req.PartitionIDs...)
@@ -250,6 +252,11 @@ func (ob *TargetObserver) check(ctx context.Context, collectionID int64) {
 	ob.keylocks.Lock(collectionID)
 	defer ob.keylocks.Unlock(collectionID)
 
+	// if collection release, skip check
+	if ob.meta.CollectionManager.GetCollection(ctx, collectionID) == nil {
+		return
+	}
+
 	if ob.shouldUpdateCurrentTarget(ctx, collectionID) {
 		ob.updateCurrentTarget(ctx, collectionID)
 	}