[TiCDC] Owner fails to clean up stale metrics leading to incorrect display in dashboard

[liuzix]

If a owner has resigned, it may fail to clean up its metrics data. In some metric items, such as changefeed checkpoint lag, the dashboard tries to calculate a maximum of all existing time series, causing incorrect displayed values in the dashboard.

For example, in the screenshot above, the checkpoint lag remains unchanged after an old owner has resigned with a high lag.