Feature Request

Describe your feature request related problem

TiDB need to get region count between startKey and endKey from /pd/api/v1/stats/region to calculate TiFlash replica progress. GetRegionStatus will scan regions in BTree so it will have heavy cost if there are many regions.
In fact, TiDB only need region count, if /pd/api/v1/stats/region can add parameter to only get region count, the cost of /pd/api/v1/stats/region will reduce from O(N) to O(logN).

1 million regions, call /pd/api/v1/stats/region per 2 seconds, flame graph on PD:

Describe the feature you'd like

Add a parameter in /pd/api/v1/stats/region to get only region count.

Describe alternatives you've considered

Teachability, Documentation, Adoption, Migration Strategy