(mongodb >=1.4.0)
MongoDB\Driver\Session::getOperationTime — Returns the operation time for this session
Returns the operation time for this session. If the session has not been used
for any operation and
MongoDB\Driver\Session::advanceOperationTime() has not
been called, the operation time will be null
此函数没有参数。
Returns the operation time for this session, or null if the session has no
operation time.