Charm is happily "active/idle" even when workload cannot reach any datasources

[sed-i]

Enhancement Proposal

Data not appearing on the Grafana panel due to a firewall blocking ports from related datasources.
This was not initially obvious because, in other deployments, the COS subnet was the same as the OpenStack management subnet, so we never ran into firewall issues. However, this time, the COS subnet was different, and it took us a while to figure this out. -- @qch2012

It could be handy if the charm blocks when the datasource URLs are not routeable/reachable. Some impl ideas:

• Check on update status, because at deploy time this check could be more racy than on update-status.
• Have a custom pebble notice running in the background that would wake up the charm on "rising/falling edge".

Very similar problem to canonical/grafana-agent-operator#57.

[simskij]

I'm not sure I agree that this is a charm concern. 🤔 Let's discuss it more with the team

[mmkay]

It's likely the same logic as we need in grafana-agent-operator, but it's not clear yet whether it's a good idea to block the Grafana charm.

[lucabello]

We should do this with Pebble checks.