Record chunked response size before sending last chunk

[DaveCTurner]

We expect that the HTTP response stats should reflect the responses
received by the client, but today we record that a chunked-encoded
response was sent after sending the last chunk, which might mean that
the client can receive the complete response body and then retrieve
stats that do not include that response. With this commit we record the
stats before sending the last chunk.

Closes #102547

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[DaveCTurner]

Calling this a >non-issue because really stats are allowed to be a little stale, it just seems easier to make them more consistent rather than make the test more lenient.

[ywangd]

LGTM

Thanks for fixing it. I am amazed how quickly you spotted the issue. Did you manage to reproduce it or did you just discover it by reading the code?

[DaveCTurner]

Did you manage to reproduce it or did you just discover it by reading the code?

I spotted that the missing response stats were always the GET _cat/nodes call, which was the last one. Pretty sure it wasn't actually getting lost, probably just delayed, so suspected a race. So then I went to check whether there was any guarantee that the stats would be updated before the client requested them, and saw the problem.

Didn't actually reproduce it, but I imagine it would happen every time with a sufficient delay just before updating the stats.