static <T> StreamMessage<T> concat(Publisher<? extends Publisher<? extends T>> publishers) {
        requireNonNull(publishers, "publishers");
        return new ConcatPublisherStreamMessage<>(of(publishers));
    }
Returns a concatenated {@link StreamMessage} which relays items of the specified {@link Publisher} of
{@link Publisher}s in order, non-overlappingly, one after the other finishes.
core/src/main/java/com/linecorp/armeria/common/stream/StreamMessage.java