void doRequestDemand(long cumulativeDemand) {
            if (upstreamSubscription == null) {
                return;
            }

            if (cumulativeDemand <= requestedDemand) {
                return;
            }

            final long delta = cumulativeDemand - requestedDemand;
            requestedDemand = cumulativeDemand;
            upstreamSubscription.request(delta);
        }
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