fix: instruct an alert rule to maintain its previous state when a datasource disconnection occurs

In Grafana, the execErrState: KeepLastState setting instructs an alert rule to maintain its previous state (e.g., Normal, Alerting, or Pending) when the underlying query execution fails or produces an unexpected error. It prevents false alarms caused by temporary network timeouts or database disconnects. In our case when core dns fails and Grafana loses connectivity with Loki datasource it will prevent it from creating a false alarm.