@Override
        public void onNext(T item) {
            requireNonNull(item, "item");
            if (isCancelled()) {
                return;
            }
            downstream.onNext(item);
            produced(1);
        }
Sets the {@link OuterSubscriber}.
This should be set before subscribing to upstream.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatPublisherStreamMessage.java