cdc: the checkpoint lag isn‘t consistent with checksum

[zeminzhou]

Bug Report

1. Describe the bug

The checkout lag watched from grafana isn't consistent with checksum which was read from src & dst tikv by client-go.

2. Minimal reproduce step (Required)

1. create changefeed
2. start to put data into src tikv
3. kill pd leader
4. watching the checkpoint lag & reading src & dst tikv checksum

3. What did you see instead (Required)

the checkpoint lag is greater than checksum time to agree

4. What did you expect to see? (Required)

the checkpoint lag is almost the same checksum time to agree

[github-actions]

This issue is stale because it has been open 30 days with no activity.