Metric for pin queue size

[neelvirdy]

Is your feature request related to a problem? Please describe. Not enough visibility into pin queue size on prod until pinning starts getting backed up.

Describe the solution you'd like Similar to content manager queue size metric, we should have one for pin queue size so we can monitor it in grafana and eventually alert on it.

CM Queue Size Metric in codebase: https://github.com/application-research/estuary/blob/18f11f67b9f9da9a9494290b9b1353be240d6230/contentmgr/replication.go#L377

CM Queue Size in Grafana: https://protocollabs.grafana.net/d/9AtDvgG7k/estuary?orgId=1&editPanel=52

cc @anjor