Expose harbor metrics

Created by: chess-knight

Metrics are exposed by several Harbor components: exporter, core, jobservice, and registry.

E.g. exporter metrics:

$ kubectl port-forward svc/harbor-harbor-exporter 8001
$ curl localhost:8001/metrics | grep harbor_health
# HELP harbor_health Running status of Harbor
# TYPE harbor_health gauge
harbor_health 1

See https://goharbor.io/docs/main/administration/metrics/ for more.

Fixes #29 (closed)