Add alert on old Kafka consumer group commit timestamp

To be notified before the offsets expire, in case a consumer group has been stopped for too long. We have had this issue a few time.

We would need to configure what consumer groups are monitored though; eg. I tend to have short-lived consumers in the swh-vlorentz- namespace.

added Metrics/monitoring Metrics/monitoring labels

Example explore query

mentioned in commit ardumont/swh-lister@42d8e24d