ddl: remove redundant backfill metric counting (#43846)

ddl/backfilling_scheduler.go

@@ -483,13 +483,11 @@ func (w *addIndexIngestWorker) HandleTask(rs idxRecResult) {
 		result.totalCount = cnt
 		result.nextKey = nextKey
 		result.err = w.checkpointMgr.UpdateCurrent(rs.id, count)
-		count = cnt
 	} else {
 		result.addedCount = count
 		result.scanCount = count
 		result.nextKey = nextKey
 	}
-	w.metricCounter.Add(float64(count))
 	if ResultCounterForTest != nil && result.err == nil {
 		ResultCounterForTest.Add(1)
 	}