ShardingTaskExecutorPoolMaxSize
1 Maximum number of outbound connections each TaskExecutor connection pool can open to any given mongod instance. 2 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
1 The number of Task Executor connection pools to use for a given mongos. 2 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
1 Maximum number of simultaneous initiating connections (including pending connections in setup/refresh state) 2 each TaskExecutor connection pool can have to a mongod instance. You can set this parameter to control 3 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
作者：华为云专家 zale
