Grafana: fix queries of block cache hit panel (#8114)
Grafana: fix queries of block cache hit panel (#8114)

Signed-off-by: Liu Cong <3a7b46daa14961f163f8f61e5298392c2fe91d46@pingcap.com>
BusyJay/tikv,BusyJay/tikv,c4pt0r/tikv,pingcap/tikv,BusyJay/tikv,BusyJay/tikv,pingcap/tikv