Add alerting on underreplicated kafka topics

We don't have any alerting when one of the replicas of our kafka topics goes down.

https://grafana.softwareheritage.org/d/swL4gbeik/kafka-overview?orgId=1&viewPanel=17&var-cluster=rocquencourt&var-instance=kafka1.internal.softwareheritage.org&from=now-30m&to=now has a query we can use to set up such an alert (this metric should be 0)