@Override
        public void onSubscribe(Subscription subscription) {
            this.subscription = subscription;
            subscriber.onSubscribe(this);
        }
For hiding Subscriber interface from MultipartDecoder.
Subscribing {@code DecodedHttpStreamMessage<BodyPart>} and delegating to external subscriber.
core/src/main/java/com/linecorp/armeria/common/multipart/MultipartDecoder.java