message.trailersDistinct
类似于 message.trailers，但没有连接逻辑，并且值始终是字符串数组，即使对于仅收到一次的标头也是如此。
仅在 'end' 事件中填充。
Similar to message.trailers, but there is no join logic and the values are
always arrays of strings, even for headers received just once.
Only populated at the 'end' event.