Grafana dashboard improvements

[discostur]

Grafana Dashboard Improvements for ClickHouse Queries

Changes

1. Make ClickHouse datasource configurable via dropdown

   • Removed hardcoded datasource naming pattern k8s-$exported_namespace-$chi
   • Changed datasource variable $db to be visible and user-selectable
   • Users can now select any ClickHouse datasource from a dropdown without naming restrictions

2. Fix deprecated window function usage

   • Replaced deprecated lagInFrame() with modern lag() window function
   • Added explicit ORDER BY clause required by the new syntax
   • Fixes compatibility with ClickHouse 25.3+ which throws error: Function runningDifference is deprecated since its usage is error-prone