private void requestUpstreamForBodyPartData0() {
            // If it's closed(cancelled), let next body part request handle it.
            if (currentExposedBodyPartPublisher != null && currentExposedBodyPartPublisher.isOpen()) {
                decoded.askUpstreamForElement();
            }
        }
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