Missing Grafana self-mon dashboard

[MichaelThamm]

In COS Lite we have a Grafana self-mon dashboard or at least a dashboard with some info about the state of Grafana. We do not have this in COS and would be a great addition to the stack. This would require:

1. Relation grafana:metrics-endpoint otelcol:metrics-endpoint
2. Dashboard: self_dashboard.json