private void doOnSubscribe(Subscription s) {
            if (cancelUpstream) {
                s.cancel();
                return;
            }
            upstreamSubscription = s;
            downstreamSubscriptions.forEach(DownstreamSubscription::invokeOnSubscribe);
        }
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