metrics: Enhance Region Health Panel

## Development Task

Currently, the region health panel needs to be enhanced, some incorrect naming and placement is easily misleading
- A lot of empty-regions will automatically be created by tiflash. Because tiflash's rules are not continuous, the gaps in between will be recognized by pd as empty. These empty-regions are actually the index space of tikv.
- Need to identify `empty region` which is terminated by tiflash.
- This situation can reproduce by creating some partitions and some tiflash replicas
- A lot of states are not about region health, so need to add a new panel to record them.