The resolution for the
CSHARP-3305 issue introduced the limit on simultaneous connection attempts. Unfortunately, this breaks some existing applications when upgrading to 2.13.x, if they operate under considerable load with high concurrency, especially when using co-hosted mongos instance.
I surely understand that you would like to prevent overloading the server with a lot of connections. We even implemented similar measures by introducing a delay in IEventSubscriber.ConnectionPoolAddingConnectionEvent handler. Unfortunately, the apparent lack of ability to disable it or configure the MongoInternalDefaults.ConnectionPool.MaxConnecting blocks the upgrade to 2.13.x for us quite possibly may affect other users.
I would like to ask you to expose the MaxConnecting parameter so driver users can tune it to their liking and disable it.