why the "rocketmq_group_count" can get the consumer group that has been deleted

**Describe the bug**
I have many consumer groups has been deleted from namesrv，then exec shell command ``` mqadmin consumerProgress -n :9876 ``` ，can not get those consumer groups， but the metric ``` rocketmq_group_count``` can get； The metric ``` rocketmq_group_diff ``` can not get those consumer groups that has been deleted，Is the right;

**To Reproduce**
Steps to reproduce the behavior:
1. ``` mqadmin deleteSubGroup -g -n :9876 -c DefaultCluster```，delete several groups;
2. ``` mqadmin consumerProgress -n :9876 ```, Confirm the result of the deletion；
3. Get the value of the metric ``` rocketmq_group_count ```, Check that those deleted groups still exist ;

**Expected behavior**
``` rocketmq_group_count ``` cannot get the deleted consumer groups;

**Desktop (please complete the following information):**
- OS: debian 10
- Browser: chrome
- Version: master