GlobalDefaultReservation to enable late setup of the global dispatch

[MarcusGrass]

Motivation

Fixes #2587

Solution

A lot of the context is in the issue, this creates a global_default guard which will prevent threads from setting its local dispatch to the none-dispatcher permanently in cases where the global default isn't found.

In the case of an application that doesn't set up a subscriber, it will immediately hit the UNINITIALIZED path of get_global_if_not_reserved() and set the thread local to the none-dispatcher immediately.
In cases where the user has reserved a global default it will defer the setting of its thread local state and temporarily use the NONE-dispatcher. Other than that it works as expected and the only downside should be code-bloat.

I would have added more tests but messing with the global static makes that pretty difficult.

E:
This is an okayish solution but it runs into a bit of an issue depending on the state of the application when the global subscriber picks up.

Another interesting solution I tried out was to just make a buffering subscriber, that's kind of a mess in itself but imagine a temporary store that's guarded the same way as this fix, but it just saves events and then replays them once the real subscriber connects. That would likely come with its own set of challenges though, since then you really have to take care to not mess up spans that may be entered but not exited when the swap is made.

[hawkw]

Hi @MarcusGrass, thanks for working on this!

It looks like this change adds a new API which users are required to call to avoid the bug described in #2587. This means that existing user code that exhibits the buggy behavior will not be fixed without user intervention. That's fine if it's the only possible solution, but I would really prefer to fix the bug without requiring users to change their existing code.

I'm going to do a bit more investigation to see if we can solve the issue without needing to add a new API, but if that doesn't seem practical, we can move forward with this change.

[hawkw]

since this API is available even when the standard library is not enabled, this should use core::mem::forget instead, to fix the compilation error with the std dependency disabled: https://github.com/tokio-rs/tracing/actions/runs/4947905935/jobs/8853850173?pr=2592#step:5:35

Suggested change

	std::mem::forget(self);
	core::mem::forget(self);

[hawkw]

The change in #2593 obviates the need for this API. Thanks for the contribution and work on debugging this issue!