code: public final Flux<T> doOnSubscribe(Consumer<? super Subscription> onSubscribe) {
  		Objects.requireNonNull(onSubscribe, "onSubscribe");
  		return doOnSignal(this, onSubscribe, null, null, null, null, null, null);
  	}
docs: Add behavior (side-effect) triggered when the {@link Flux} is subscribed.
  <p>
  This method is <strong>not</strong> intended for capturing the subscription and calling its methods,
  but for side effects like monitoring. For instance, the correct way to cancel a subscription is
  to call {@link Disposable#dispose()} on the Disposable returned by {@link Flux#subscribe()}.
  <p>
  <img class="marble" src="doc-files/marbles/doOnSubscribe.svg" alt="">
  
  @param onSubscribe the callback to call on {@link Subscriber#onSubscribe}
  
  @return an observed  {@link Flux}