Usually it is the ReplicaSetMonitor that tracks a shard's replica set nodes and notifies the shard registry if nodes are added or removed. However, in a scenario where a node's RSM loses track of the replica but is able to discover new nodes through config.shards, having a mechanism to inform the RSM of a new node would improve availability. SERVER-115646 was filed for this purpose, once it is done, the shard registry should make use of it.