Support mirroring FiberRef contents to ThreadLocal

It would be really useful for FiberRef contents to be mirrored to non-zio infra (outside of our control) backed by ThreadLocals. For example, when using NewRelic and it’s Tokens to connect async flows to what they call a transaction (think an incoming request). In this case, we would store the new relic token in a FiberRef like this:

    FiberRef.make(nrToken, link = _.link())

The provided link function will be called by zio after every async boundary so any effect running in this fiber (or a fiber inheriting this ref) will run on a thread “linked” to the token. There might also be a need for unlink function to clear the the thread at the end.

(MDC is a simpler example, but maybe easier to work around with libraries like zio-logging or FiberRef.unsafeAsThreadLocal)

I’ve created this Draft PR a while back with a naive implementation for this.

@adamgfraser, @jdegoes WDYT?

[dkarlinsky]

Thanks @jdegoes !

[jdegoes]

I wouldn’t do it when creating fiber ref, but do it separately. There’s nothing necessarily connected to fiber ref in such a feature. It may be more connected with Supervisor. Let me think about this for a week, but I do absolutely think we can get this into ZIO (thanks for the suggestion).