Report migrated tuples count in total

What does this MR do?

Report the total count of all tuples already migrated by the migration. There's a bit of overlap with the batched_migration_job_updated_tuples_total counter, however in this case we're interested in all migrated tuples since the beginning of the migration (which may span a long time). Hence - we just report this as a gauge here.

Does this MR meet the acceptance criteria?

Conformity

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