code: public static Function<Flux<Long>, Publisher<?>> instant() {
          return iterations -> iterations
              .flatMap(iteration -> Mono
                  .just(0L)
                  .doOnSubscribe(logDelay(Duration.ZERO)), 1);
      }
docs: Implements an instant (no delay) for use with {@link Mono#repeatWhenEmpty(Function)}
  
  @return an instant (no delay) {@link Publisher}