@Override
        public void subscribe(Subscriber<? super T> subscriber, EventExecutor executor) {
            subscribe(subscriber, executor, false, false);
        }
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