新增于: v9.4.0
此属性包含队列中准备写入的字节数（或对象数）。
该值提供有关 highWaterMark 状态的内省数据。
Added in: v9.4.0
This property contains the number of bytes (or objects) in the queue
ready to be written. The value provides introspection data regarding
the status of the highWaterMark.