事件：'wantTrailers'

【Event: 'wantTrailers'】

'wantTrailers' 事件在 Http2Stream 已将最终的 DATA 帧排队准备发送，并且 Http2Stream 已准备好发送尾随头时触发。在发起请求或响应时，必须将 waitForTrailers 选项设置为该事件才能被触发。

【The 'wantTrailers' event is emitted when the Http2Stream has queued the final DATA frame to be sent on a frame and the Http2Stream is ready to send trailing headers. When initiating a request or response, the waitForTrailers option must be set for this event to be emitted.】