@Override
        public void onNext(Publisher<? extends T> publisher) {
            requireNonNull(publisher, "publisher");
            final StreamMessage<? extends T> streamMessage = StreamMessage.of(publisher);
            currentStreamMessage = streamMessage;
            inInnerOnSubscribe = true;
            streamMessage.subscribe(innerSubscriber, executor, innerSubscriber.options);
        }
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