fix(grafana/dashboard): dashboard to use datasource variable

## Bug Reproduction Prerequisites
The bug could be observed exclusively in scenarios involving multiple Prometheus datasources.

## Root Cause Analysis
Upon importing a dashboard, Grafana replaces all `${DS_PROMETHEUS}` placeholders with the specified datasource. Issue arises due to the presence of a dashboard variable named `DS_PROMETHEUS`. Grafana substitutes variable invocations with the provided datasource, effectively hardcoding it during the import process. Consequently, if a user later selects a different datasource, this change does not impact the dashboard since the datasource value was **hardcoded during the initial import**.

## Proposed Fix
The proposed solution eliminates the reliance on the DS_PROMETHEUS variable as an input during the import process. Instead, the fix involves depending on the provided variable, ensuring a more dynamic and adaptable integration with various datasources post-import.

deploy/grafana/dashboards/nginx.json

@@ -1,14 +1,4 @@
 {
-  "__inputs": [
-    {
-      "name": "DS_PROMETHEUS",
-      "label": "Prometheus",
-      "description": "",
-      "type": "datasource",
-      "pluginId": "prometheus",
-      "pluginName": "Prometheus"
-    }
-  ],
   "__requires": [
     {
       "type": "grafana",