(mongodb >=1.11.0)
MongoDB\Driver\Monitoring\CommandStartedEvent::getServiceId — Returns the load balancer service ID for the command
When the driver is connected to a MongoDB cluster through a load balancer,
the service ID corresponds to the serviceId field in the
hello command response.
此函数没有参数。
Returns the load balancer service ID, or null if the driver is not
connected to a load balancer.