Add support for tracking Akka.Cluster.Sharding `ShardRegion` metrics

[Aaronontheweb]

We should add support for:

• [x] Enumerating all ShardRegions by name
• [x] Tracking the number of shards / entities per ShardRegion
• [x] Tracking the shard / entities per ShardRegion per Address / role

I think this should be pretty straightforward to do within the Phobos.Actor.Cluster module.

[Aaronontheweb]

We've added dashboard support for this in Prometheus and will do the same for other supported platforms: https://github.com/petabridge/phobos-dashboards/pull/22