many users have more than 1 db cluster because they process data with different requirements (small &
very reliable vs big & transient).
In one instance a user has 7 clusters, which means they need to spawn 21 config servers, which is pain for admin and backup.
It would be beneficial to be able to share config servers between clusters since the data & rps requirement is low.
For example instead of the "config" db name, the name could include the cluster name and each cluster would have its own db.
Cluster name can be passed to mongos command along the configdb hostnames.