Region Health panel missing data if PD suffers from leader transfer

[iosmanthus]

Bug Report

What did you do?

# create a 3-PD cluster
> curl -s http://<leader>/metrics | grep empty-region-count
pd_regions_status{type="empty-region-count"} 14
> pd-ctl member leader transfer <another-leader>
> pd-ctl member leader transfer <leader>
> curl -s http://<leader>/metrics | grep empty-region-count

The last command outputs nothing, meaning the metrics are missing.

What did you expect to see?

pd_regions_status{type="empty-region-count"} 14

What did you see instead?

nothing

What version of PD are you using (pd-server -V)?

master

[rleungx]

introduced by #5802