Add DS_PROMETHEUS datasource for templating

When running this dashboard on Grafana, I received the error `Datasource named ${DS_PROMETHEUS} was not found`, this change fixes that issue.

deploy/grafana/dashboards/request-handling-performance.json

@@ -911,6 +911,16 @@
   ],
   "templating": {
     "list": [
+      {
+        "hide": 0,
+        "label": "datasource",
+        "name": "DS_PROMETHEUS",
+        "options": [],
+        "query": "prometheus",
+        "refresh": 1,
+        "regex": "",
+        "type": "datasource"
+      },
       {
         "allValue": ".*",
         "current": {},