Stream & Function<Flux<?>, Flux<?>> where ? is NOT a message breaks

[marcingrzejszczak]

Having such a function

	static class FluxToFluxFunction implements Function<Flux<String>, Flux<String>> {

		private static final Logger log = LoggerFactory.getLogger(FluxToFluxFunction.class);

		static final Scheduler SCHEDULER = Schedulers.newParallel("sleuthFunction");

		@Override
		public Flux<String> apply(Flux<String> input) {
			return input.doOnEach(signal -> log.info("Got a message"))
					.flatMap(s -> Mono.delay(Duration.ofMillis(1), SCHEDULER).map(aLong -> {
						log.info("Logging [{}] from flat map", s);
						return s.toUpperCase();
					}));
		}

	}

causes cast exceptions when adding tracing.