How do I access response content length in a middleware?

[khuongduybui]

This is a duplicate of #829 in case somebody has a better workaround.

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I'm creating an access log writer middleware (https://github.com/khuongduybui/fresh-logging) but I'm struggling to get the content length for the bytes part of the common log format.

I first looked in the Content-Length response header, but it's not being set.

My next attempt was to clone the response, read its body into an ArrayBuffer, and get the byteLength.
That worked, but I found res.clone() to take anywhere from 10 to 100 millisecond for a super simple route, which is SIGNIFICANT.

I am wondering if there is a more efficient way to obtain response content length without having to clone and read.
Perhaps it could be set somewhere earlier in the chain while creating the response?