Add praefect replication delay metric

Per demo on 3/10, a useful metric would be the total time from when a replication job is created to when it is completed. This would give us total "replication delay".

Additionally, this metric should be graphed in Grafana: https://dashboards.gitlab.net/d/8EAXC-AWz/praefect?orgId=1&from=now-24h&to=now