Alert on services restarting too often

We had Loki v2.4.0 restarting every 10 minute due to a bug.

node_exporter has a option --collector.systemd.enable-restarts-metrics Enables service unit metric service_restart_total which would allow us to create a alert if a service is restarting too often.

See also: https://github.com/prometheus/node_exporter/issues/1790