@Override
        public void onNext(BodyPart bodyPart) {
            requireNonNull(bodyPart, "bodyPart");
            if (alloc != null) {
                bodyPartFutures.add(bodyPart.aggregateWithPooledObjects(alloc));
            } else {
                bodyPartFutures.add(bodyPart.aggregate());
            }
        }
Returns a random boundary used for encoding multipart messages.
core/src/main/java/com/linecorp/armeria/common/multipart/DefaultMultipart.java