code: public static Function<Flux<Long>, Publisher<?>> fixed(Duration duration) {
          return iterations -> iterations
              .flatMap(iteration -> Mono
                  .delay(duration)
                  .doOnSubscribe(logDelay(duration)), 1);
      }
docs: Implements an fixed delay for use with {@link Mono#repeatWhenEmpty(Function)}
  
  @param duration the duration of the delay
  @return a delayed {@link Publisher}