log_backup: add some hint about the slow region

[YuJuncen]

Enhancement

For now, the slow region hint of log backup only provides the range info. It is pretty hard to find the region contains that range in a huge cluster (We need run pd-ctl or something, also note that PD ctl doesn't support querying history data.)
It will be hard to debug checkpoint stuck issues: usually we need to fetch all TiKV's logs and query the start key of some region in ALL logs(!). It is painful.

We can fetch the region metadata once there are some slow regions. That will be pretty lightweight.

[BornChanger]

/component br

[BornChanger]

/label affects-6.5

[BornChanger]

/label affects-7.1

[BornChanger]

/label affects-7.5