ensure generateId executes on the scheduler thread specified by publishOn

[qnnn]

Hello @spencergibb, I recently came across this code, which is intended to prevent the execution of uuid generate on the main thread. However, after applying the map function, the publisher returned by publishOn is no longer a MonoSubscribeOnCallable, but rather a MonoPublishOn. As a result, the first execution of idGenerator::generateId actually still occurs on the thread that subscribes to it.

spring-cloud-gateway/spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/route/CompositeRouteDefinitionLocator.java

Lines 67 to 69 in 0affb76

	protected Mono<String> randomId() {
	return Mono.fromSupplier(idGenerator::generateId).map(UUID::toString).publishOn(Schedulers.boundedElastic());
	}