@Override
        public void onSubscribe(Subscription subscription) {
            requireNonNull(subscription, "subscription");
            if (upstream == null) {
                upstream = subscription;
                subscription.request(1);
            } else {
                subscription.cancel();
            }
        }
Relay items in order from a {@link StreamMessage} of {@link StreamMessage}s as a single {@link StreamMessage}
source.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatPublisherStreamMessage.java