private void subscribe(Subscriber<? super T> subscriber, EventExecutor executor,
                               boolean withPooledObjects, boolean notifyCancellation) {
            requireNonNull(subscriber, "subscriber");
            requireNonNull(executor, "executor");
            final DownstreamSubscription<T> subscription = new DownstreamSubscription<>(
                    this, subscriber, processor, executor, withPooledObjects, notifyCancellation);

            if (!subscribe0(subscription)) {
                final DownstreamSubscription<T> oldSubscription = this.subscription;
                assert oldSubscription != null;
                failLateSubscriber(executor, subscriber, oldSubscription.subscriber());
            }
        }
Returns the default {@link EventExecutor} which will be used when a user subscribes to a child
stream using {@link StreamMessage#subscribe(Subscriber, SubscriptionOption...)}.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java