feat: add undici:body:consumed diagnostics channel

[KhafraDev]

Fixes #1342

Superseded #1344 with a slightly more reliable solution (relies on consuming via the body mixins).

[codecov-commenter]

Codecov Report

Merging #1369 (6741636) into main (ae369b2) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1369      +/-   ##
==========================================
+ Coverage   94.25%   94.26%   +0.01%     
==========================================
  Files          45       45              
  Lines        4211     4222      +11     
==========================================
+ Hits         3969     3980      +11     
  Misses        242      242              

Impacted Files	Coverage Δ
lib/api/api-request.js	100.00% <100.00%> (ø)
lib/api/readable.js	89.76% <100.00%> (+0.97%)	⬆️
lib/client.js	97.93% <100.00%> (ø)
lib/core/request.js	98.65% <100.00%> (ø)
lib/handler/redirect.js	94.93% <100.00%> (ø)

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[ronag]

I'm sorry but I don't like this. Can't you just have a event for each body chunk? and then trailers will signal end of body?

[KhafraDev]

I can look into it, but this is also opt-in (rather than the old PR where it was added onto the trailers channel), and it only sends the body that was already consumed, rather than duplicating it.

Also publishing a message for each chunk would make it very hard to group together correctly, since 2 requests to the same origin/path could be made at once.

edit: also in the code I used channels.trailers when it's not part of the trailers channel, but its own separate channel (whoops). If there are no subscribers to this channel, nothing is sent. I also don't think it would be bad for performance because it doesn't do anything under the hood, other than pass the body to the channel, which was already consumed by the user.

import { request } from 'undici'
import { text } from 'stream/consumers'

const { body } = await request('...') // nothing is published

await body.text() // a message is published, identical to the value this method returned

await text(body) // no message published

for await (const chunk of body) {
  // ...
}

// no message published

[mcollina]

lgtm, good work

[KhafraDev]

I'm getting mixed signals here 😅

[mcollina]

I'm sorry but I don't like this. Can't you just have a event for each body chunk? and then trailers will signal end of body?

I have a feeling this could impact perf in the wrong way as that's a fairly hot path. Anyway if that does not regress the perf, it would be ok.

[KhafraDev]

I think this way is more convenient and faster. It also only requires 1 message rather than x amount of chunks received and doesn't require the user to parse the body themselves.