Improve incremental snapshot metrics

Currently incremental snapshots do not populate the Remaining Table Count metric which is a big metric to measure level of progress for the snapshot operation

Also, with the new snapshot mechanism it is possible to take a good enough measurement of percentage of completeness per table since the start and end points are known. Caveat that this should only work for integer primary keys and can't take into account non contiguous space blocks of ids, i.e., deleted records. Maybe not expose this percentage at all but expose start end and current Ids of the snapshot.