Move debuglog out of diagnostic channels

[mcollina]

Unfortunately, using diagnostic channels to set up debuglog was a bad idea. It won't work because our implementation inside Node.js will react to the same NODE_DEBUG flag, and duplicate the messages, making these logs hard to read.

A solution for this would be:

1. detect if we are running in Node.js core, and if so, use a different debuglog prefix
2. move away from using diagnostics channels and just inline the debuglogs

Do you see any other options?

[metcoder95]

I'd prefer to see if we can inline it. The coupling of both concepts seems like a good idea but in reality, they overlap and serve different purposes.

[KhafraDev]

detect if we are running in Node.js core, and if so, use a different debuglog prefix

this was my idea, haven't had time to work on it. It's easy to detect when we're in node core already

undici/lib/web/fetch/index.js

Lines 68 to 70 in fe1f132

	const defaultUserAgent = typeof __UNDICI_IS_NODE__ !== 'undefined' || typeof esbuildDetection !== 'undefined'
	? 'node'
	: 'undici'

[Uzlopak]

I am curious if not PR #3600 introduced this issue. Maybe the old subscribe referred to a separate diagnostic sub channel or so? I mean we had not this issue before?!

[mcollina]

The issue was always there, but we avoided it by disabling global fetch in our tests. This is not possible anymore.