metrics: rename Operator limit (tikv#4605)

* rename, ref tikv#4399

Signed-off-by: lhy1024 <[email protected]>

* address comment

Signed-off-by: lhy1024 <[email protected]>

Co-authored-by: 混沌DM <[email protected]>
Co-authored-by: Ti Chi Robot <[email protected]>

Author: 3 people

metrics/grafana/pd.json

@@ -3015,7 +3015,7 @@
           "timeFrom": null,
           "timeRegions": [],
           "timeShift": null,
-          "title": "Operator limit",
+          "title": "Schedulers reach limit",
           "tooltip": {
             "shared": true,
             "sort": 0,