reactor-core/src/main/java/reactor/core/publisher/Flux.java
public final Flux<T> doOnSubscribe(Consumer<? super Subscription> onSubscribe) {
		Objects.requireNonNull(onSubscribe, "onSubscribe");
		return doOnSignal(this, onSubscribe, null, null, null, null, null, null);
	}
public final Flux<T> doOnSubscribe(Consumer<? super Subscription> onSubscribe) {
		Objects.requireNonNull(onSubscribe, "onSubscribe");
		return doOnSignal(this, onSubscribe, null, null, null, null, null, null);
	}
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}