backfill metrics should be dropped after backill finishes

[hzxa21]

We found that "Backfill Snapshot Read Throughput" metric will continuously report 0 for tables that finish backfilling and for table that are already dropped. I think same things apply to other backfill metrics relevant to no-shuffle backfill, arrangement backfill and cdc backfill. It is better to use LabelGuardedMetric here.

Relevant issue: #14838