@Override
        public void onSubscribe(Subscription subscription) {
            requireNonNull(subscription, "subscription");
            if (isCancelled()) {
                subscription.cancel();
                return;
            }
            // Reset 'completed' to subscribe to new Publisher
            currentPublisherCompleted = false;
            setUpstreamSubscription(subscription);
            assert outerSubscriber != null;
            outerSubscriber.inInnerOnSubscribe = false;
        }
Sets the {@link OuterSubscriber}.
This should be set before subscribing to upstream.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatPublisherStreamMessage.java