Add alerts for network health relays
We have some relays and their metrics scraped via Prometheus (namely akka, ukko, d2d4, and Najdorf). We should work on alerts so we can avoid stumbling into some or all of the metrics not being exposed anymore in our dashboards due to $issue (as witnessed in tpo/network-health/team#281 (closed)). /cc @hiro, @dgoulet