ShardingTaskExecutorPoolMaxSize
Maximum number of outbound connections each TaskExecutor connection pool can open to any given mongod instance.
The maximum possible connections to any given host across all TaskExecutor pools is
连接池最大大小，默认没有限制
对于mongod最大连接控制：ShardingTaskExecutorPoolMaxSize * taskExecutorPoolSize
ShardingTaskExecutorPoolMinSize
Minimum number of outbound connections each TaskExecutor connection pool can open to any given mongod instance.
连接池最小大小，默认为1
taskExecutorPoolSize
The number of Task Executor connection pools to use for a given mongos.
The parameter has a minimum value of 4 and a maximum value of 64.
任务池大小，默认是cpu核数
ShardingTaskExecutorPoolMaxConnecting
Maximum number of simultaneous initiating connections (including pending connections in setup/refresh state)
each TaskExecutor connection pool can have to a mongod instance. You can set this parameter to control
the rate at which mongos adds connections to a mongod instance.
并发进行初始连接的数量，默认没有限制，该值保持在ShardingTaskExecutorPoolMaxSize以下
ShardingTaskExecutorPoolRefreshRequirementMS
Maximum time the mongos waits before attempting to heartbeat a resting connection in the pool.
连接池保活心跳的间隔时间，默认1分钟
ShardingTaskExecutorPoolRefreshTimeoutMS
Maximum time the mongos waits for a heartbeat before timing out the heartbeat
连接池保活心跳的超时时间，默认20秒
ShardingTaskExecutorPoolHostTimeoutMS
Maximum time that mongos goes without communication to a host before mongos drops all connections to the host.
空闲时间阈值，默认30秒，当mongos与mongod在指定时间内没有交互时会释放连接。
参考文档
https://docs.mongodb.com/v3.4/reference/parameters/#param.ShardingTaskExecutorPoolMaxSize