Report overall per-table progress of primary key migration
In order to simplify getting status updates on the primary key migration effort, we'd like to report the overall progress of the primary key migration to prometheus.
That is, we follow this pattern:
- We keep track of the estimated relation size at the beginning of the migration #326994 (closed)
- We continuously report the % complete when a job completes
The % complete can be calculated by the following ratio:
- Sum of the batch sizes from previous, completed jobs
- Estimated relation size at the beginning of the migration