Online DDL: progress & ETA for Vreplication migrations

[shlomi-noach]

Description

#7980 introduced VReplicaiton tracking for rows_copied. This PR both intensifies that tracking, and follows up in Online DDL: the online DDL executor periodically checks rows_copied from _vt.vstream, compares with estimated table_rows, looks at started_timestamp, and generates values for progress and for eta_seconds.

Online DDL does so every 1 minute, normally, which means the valuee progress and eta_seconds are estimates only. Plus, vcopier itself only updates rows_copied every 30s, so we could get almost a 90s stale value for progress and eta_seconds. Which is acceptable to us at this tim.

Related Issue(s)

• VReplication: evaluate "progress" and "ETA" for online DDL #7632
• Online DDL: tracking issue #6926
• followup to VReplication: tracking rows_copied #7980

Checklist

• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[shlomi-noach]

This PR is generally good to go. I'm struggling if/whether to add endtoend test that checks this specific behavior.

Obviously it would be nice to test the behavior. But then, I'm not sure how to go about it. It would take a relatively long migration to be able to test this. We'd need a migration that is absolutely known to run for over 90sec, and that we can ensure will show a non-zero, non-100% progress value, at some point. This can be done, at a cost. It will take long minutes to populate and test such a migrated table, and I'm concerned about impact to CI.

[shlomi-noach]

Now also complements #8255 and updates rows_copied for VReplication migrations. Note that while migration is running rows_copied is only updated once per minute, and based on vreplication's rows_copied which in itself can be delayed.

When the migration completes, rows_copied is ensured to be the final & exact value.

[shlomi-noach]

This is now ready for review.

[rohit-nayak-ps]

lgtm