[pawelsadlo]

Needed for #395 as a tool to propagate opentelemetry context to forks

This is OpenTelemetry agnostic approach that enables propagation of open telemetry context.

Scope gets a new ScopeLocal field "Fork_decorator" which can be used to decorate fork creation logic.

decoration is invoked when fork() method is called (not when forked thread runs it)

Why its needed for OpenTelemetry context propagation:

Threads are created inside supervised block by thread which runs supervised, not by the one which calls fork.

Therefore propagation approach used in ox, via (virtual) thread factory doesn't work, because all threads (and their threadLocals) are children of one thread, even though they are syntactically nested.

#395 introduces

public enum PropagatingForkDecorator implements Decorator {
    INSTANCE;

    @Override
    public <T> Callable<T> decorate(Callable<T> f) {
        // captured on the thread which starts the fork, where the context is still current
        Context forkingContext = Context.current();
        return () -> {
            try (Scope ignored = forkingContext.makeCurrent()) {
                return f.call();
            }
        };
    }
}

which can be bound to scopelocal value of Fork_decorator to propagate open telemetry context properly.

ScopeLocal storage of Decorator enables this feature with minimal api change