code: public static Flux<Long> interval(Duration delay, Duration period) {
  		return interval(delay, period, Schedulers.parallel());
  	}
docs: Create a {@link Flux} that emits long values starting with 0 and incrementing at
  specified time intervals, after an initial delay, on the global timer. If demand is
  not produced in time, an onError will be signalled with an
  {@link Exceptions#isOverflow(Throwable) overflow} {@code IllegalStateException}
  detailing the tick that couldn't be emitted. In normal conditions, the {@link Flux}
  will never complete.
  <p>
  Runs on the {@link Schedulers#parallel()} Scheduler.
  <p>
  <img class="marble" src="doc-files/marbles/intervalWithDelay.svg" alt="">
  
  @param delay  the {@link Duration} to wait before emitting 0l
  @param period the period {@link Duration} before each following increment
  
  @return a new {@link Flux} emitting increasing numbers at regular intervals