onHeaders listner not invoking when a response is about to write headers

[pg2203]

We are recording start and end time when the request is served, so we start recording time in postRouting and try to end recording just when a response is about to write headers. For this i am using onHeaders library (https://github.com/jshttp/on-headers) where we can override onHeaders listener, but it looks like it never invokes this listener. I suspect when it reaches to postController the header is already sent. please suggest