Report batched migration progress

What does this MR do?

This adds a prometheus gauge to report the estimated progress made for a batched migration. This is based on summing up all the individual batch sizes from succeeded jobs and comparing that to the total_tuple_count estimation for the migration.

Does this MR meet the acceptance criteria?

Conformity

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  • I have included a changelog entry.
  • I have not included a changelog entry because "hidden behind feature flag still".
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