Container Registry: Increase number for online DB replicas for redundancy

Context

Currently the registry DB cluster has a single online replica, with other replicas being offline for disaster recovery/backup reasons.

As we completed the rollout of the DB load balancing feature for the container registry application, we're now shifting the vast majority of traffic to the replica host (source).

Ask

For redundancy and availability reasons, we should consider increasing the number of online replicas from 1 to 2.

The registry performs DB load balancing by performing DNS lookups against the registry-db-replica.service.consul. consul DNS record, which currently returns 3 hosts (PgBouncer, same IP, 3 different ports). A new replica should be advertised in the same way for transparent adoption by the registry application.