@Override
        public void onSubscribe(Subscription subscription) {
            if (cancelled()) {
                subscription.cancel();
            } else {
                setUpstreamSubscription(subscription);
            }
        }
Relay items in order from subsequent {@link StreamMessage}s as a single {@link StreamMessage} source.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatArrayStreamMessage.java