mvcc: add TODOs

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

mvcc/metrics_txn.go

@@ -50,18 +50,18 @@ func (tw *metricsTxnWrite) End() {
 	defer tw.TxnWrite.End()
 	if sum := tw.ranges + tw.puts + tw.deletes; sum > 1 {
 		txnCounter.Inc()
-		txnCounterDebug.Inc()
+		txnCounterDebug.Inc() // TODO: remove in 3.5 release
 	}
 
 	ranges := float64(tw.ranges)
 	rangeCounter.Add(ranges)
-	rangeCounterDebug.Add(ranges)
+	rangeCounterDebug.Add(ranges) // TODO: remove in 3.5 release
 
 	puts := float64(tw.puts)
 	putCounter.Add(puts)
-	putCounterDebug.Add(puts)
+	putCounterDebug.Add(puts) // TODO: remove in 3.5 release
 
 	deletes := float64(tw.deletes)
 	deleteCounter.Add(deletes)
-	deleteCounterDebug.Add(deletes)
+	deleteCounterDebug.Add(deletes) // TODO: remove in 3.5 release
 }