Add ThreadLocalAccessor to propagate Baggage with Reactor

[jonatan-ivanov]

Example that should work:

@RequestMapping("/")
Mono<String> baggageTest() {
    try (BaggageInScope scope = this.tracer.createBaggageInScope("test", "42")) {
        return webClient.get().uri("/").retrieve().bodyToMono(String.class);
    }
}

[marcingrzejszczak]

For Tracing 1.0.x I've written such a test

@Test
    void baggageWithContextPropagation() {
        Hooks.enableAutomaticContextPropagation();

        Span span = tracer.nextSpan().start();
        try (Tracer.SpanInScope spanInScope = tracer.withSpan(span)) {
            try (BaggageInScope scope = this.tracer.createBaggage(KEY_1, VALUE_1).makeCurrent()) {
                String baggageOutside = this.tracer.getBaggage(KEY_1).get();
                then(baggageOutside).isEqualTo(VALUE_1);
                log.info("BAGGAGE OUTSIDE OF REACTOR [" + baggageOutside + "], thread [" + Thread.currentThread() + "]");
                Baggage baggageFromReactor = Mono.just(KEY_1)
                    .publishOn(Schedulers.boundedElastic())
                    .flatMap(s -> Mono.just(this.tracer.getBaggage(s))
                        .doOnNext(baggage -> log.info("BAGGAGE IN OF REACTOR [" + baggageOutside + "], thread [" + Thread.currentThread() + "]")))
                    .block();
                then(baggageFromReactor).isNotNull();
                then(baggageFromReactor.get()).isEqualTo(VALUE_1);
            }
        }
    }

And it's passing 🤷 , logs

Aug 18, 2023 1:30:27 PM io.micrometer.tracing.otel.bridge.BaggageTests baggageWithContextPropagation
INFO: BAGGAGE OUTSIDE OF REACTOR [value1], thread [Thread[Test worker,5,main]]
Aug 18, 2023 1:30:27 PM io.micrometer.tracing.otel.bridge.BaggageTests lambda$baggageWithContextPropagation$1
INFO: BAGGAGE IN OF REACTOR [value1], thread [Thread[boundedElastic-1,5,main]]

It's working both for OTel and Brave.

UPDATE: It's working cause get() is called in the same thread as Reactor...