Add metrics, API and processing logic for Disconnect Region

## Feature Request

### Describe your feature request related problem

When the Region cannot elect a Leader due to some problems, the Region will not be able to send a heartbeat to the PD. The relevant information in the PD will always maintain the information of the last heartbeat. There are currently no statistics for this type of Region.

### Describe the feature you'd like

Add `Disconnect Region` statistics in `Region Healthy` and add related APIs.

### Describe alternatives you've considered

### Teachability, Documentation, Adoption, Migration Strategy