Documentation/op-guide: highlight db size metrics change

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

Documentation/op-guide/maintenance.md

@@ -149,7 +149,9 @@ $ ETCDCTL_API=3 etcdctl put newkey 123
 OK
 ```
 
-The metric `etcd_debugging_mvcc_db_total_size_in_use_in_bytes` indicates the actual database usage after a history compaction, while `etcd_debugging_mvcc_db_total_size_in_bytes` shows the database size including free space waiting for defragmentation. The latter increases only when the former is close to it, meaning when both of these metrics are close to the quota, a history compaction is required to avoid triggering the space quota.
+The metric `etcd_mvcc_db_total_size_in_use_in_bytes` indicates the actual database usage after a history compaction, while `etcd_debugging_mvcc_db_total_size_in_bytes` shows the database size including free space waiting for defragmentation. The latter increases only when the former is close to it, meaning when both of these metrics are close to the quota, a history compaction is required to avoid triggering the space quota.
+
+`etcd_debugging_mvcc_db_total_size_in_bytes` is renamed to `etcd_mvcc_db_total_size_in_bytes` from v3.4.
 
 ## Snapshot backup