Wait for all internal discoveries are done before exiting. (#4508)

Signed-off-by: Yecheng Fu <cofyc.jackson@gmail.com>

discovery/kubernetes/kubernetes.go

@@ -370,6 +370,8 @@ func (d *Discovery) Run(ctx context.Context, ch chan<- []*targetgroup.Group) {
 	}
 
 	d.Unlock()
+
+	wg.Wait()
 	<-ctx.Done()
 }