Update dashboards when relation broken (#285)

Previously, when the relation between grafana-agent
and a charm providing a dashboard
(over the grafana-dashboards-consumer relation)
was broken, the dashboards would not be removed.
This fixes that issue, by also running the dashboards changed method
when the relation is broken.

src/charm.py

@@ -88,6 +88,10 @@ def __init__(self, *args):
             self.on["grafana-dashboards-consumer"].relation_changed,
             self._on_dashboards_changed,
         )
+        self.framework.observe(
+            self.on["grafana-dashboards-consumer"].relation_broken,
+            self._on_dashboards_changed,
+        )
 
         self.framework.observe(
             self.on.agent_pebble_ready,  # pyright: ignore