ReactiveDiscoveryClient.probe is practically a no-op

[trajano]

Describe the bug

the .probe method is defined as

default void probe() {
    getServices();
}

and getServices() also returns a Flux, since Flux don't do anything without a subscribe this is practically a no-op

Perhaps the interface should be defined as returning Mono<Void> as in

default Mono<Void> probe() {
    return getServices()
      .then();
}