grafana overview storage capacity panel display anomaly

## Bug Report

### What did you do?

overview -> pd -> storage capacity / current storage size / number of regions display anomaly

Reproduction steps :
`tiup playground v8.5.5 --db 1 --kv 2 --pd 1`

From 8.5.5, pd_cluster_status has added a "store" label . When there are more than one stores in cluster, it will return multiple records, causing display abnormalities.

### What did you expect to see?
As shown before v8.5.5

### What did you see instead?
“Only queries that return single series/table is supported”

### What version of PD are you using (`pd-server -V`)?
v8.5.5