Add Prometheus recording and alerting rules and Grafana dashboards

[metalmatze]

I've created some recording and alerting rules based on the metrics we get when monitoring a CockroachDB instance.
Going forward I want to add these as a mixin the project and then even utilize them in the generated YAML and Operator to even apply them as PrometheusRule resources to the cluster.
Dashboards need to be applied manually or imported via jsonnet.