Documentation/op-guide: promote db size metric in grafana.json

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

Documentation/op-guide/grafana.json

@@ -341,7 +341,7 @@
                         "steppedLine": false,
                         "targets": [
                             {
-                                "expr": "etcd_debugging_mvcc_db_total_size_in_bytes{job=\"$cluster\"}",
+                                "expr": "etcd_mvcc_db_total_size_in_bytes{job=\"$cluster\"}",
                                 "hide": false,
                                 "interval": "",
                                 "intervalFactor": 2,