@Override
        public void onComplete() {
            if (isCancelled()) {
                return;
            }
            currentPublisherCompleted = true;
            assert outerSubscriber != null;
            if (outerSubscriber.completed) {
                downstream.onComplete();
            } else {
                outerSubscriber.nextSource();
            }
        }
Sets the {@link OuterSubscriber}.
This should be set before subscribing to upstream.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatPublisherStreamMessage.java