Documentation Request Summary:

Adding in a new sharding task executor pool set parameter.

The parameter controls the number of simultaneous connections a pool can initiate to a host. The default remains unlimited, while smaller finite values can prevent a mongos from bursting down a mongod in the event of intermittent slowdowns and disconnects (networking, step down, etc)

Engineering Ticket Description:

A new connection pool option which controls that rate at which we add new connections. It changes behavior so that only N connections can be in the processing state at any one time (in setup/refresh). More connections will be added, if needed, as each new connection completes,
fails or times out.

available in mongos via ShardingTaskExecutorPoolMaxConnecting, defaults to unlimited.