Comment 1 for bug 2043758

By looking into the code, it looks like the charm is supposed to download the charm resource from the Juju controller into:
/var/lib/juju/agents/unit-ceph-mon-*/charm/files/prometheus_alert_rules/alert.yaml.rules

But there was no file there in any of the units.

However, explicitly running the upgrade-charm hook will call self.metrics_endpoint.update_alert_rules() and the file "alert.yaml.rules" will show up and the relation data is populated.

$ juju exec -u ceph-mon/leader "JUJU_DISPATCH_PATH=hooks/upgrade-charm ./dispatch"

I'm not sure why the initial update_alert_rules() is not called or processed.