The SERVER-44152 is a good solution to prevent mongos accepting connections until it is "warm" in this particular aspect. 

In the future, we want this to happen in a more organized way. The PM-2364 remove faulty Mongos (under development) has the concept of basic startup health checks to have some confidence that new mongos is functioning until it starts accepting new connections. As such, it also has concept of "initial health check" state when clients are not allowed.

All existing "warm-up" logic should be consolidated under this umbrella for 5.1 release and later.