DLB: Include replicas in exported DB metrics

Context

This is part of Container Registry: Database Load Balancing (DLB) (&8591 - closed). See the epic description and the listed implementation plan for more details.

Task

Make sure to include replicas in the exported DB stats metrics (https://gitlab.com/gitlab-org/container-registry/-/blob/bef20bf48c66bddb424e4f8d052aed071fed2679/registry/handlers/app.go#L423-423). We're currently only exposing metrics for the primary host. Ideally we'll use the exact same metric but add a host_type and host_id/addr label to it to differentiate between primary and replicas. This needs to be implemented within the DBLoadBalancer entity as we'll have to periodically register/unregister collectors for individual replicas added/removed to/from the pool within ResolveReplicas.