@Override
        public void onSubscribe(Subscription subscription) {
            requireNonNull(subscription, "subscription");
            if (upstream == null) {
                upstream = subscription;
                initialize();
            } else {
                subscription.cancel();
            }
        }
Returns a new {@link DecodedStreamMessage} with the specified {@link HttpDecoder},
{@link ByteBufAllocator} and {@code byteBufConverter}.
core/src/main/java/com/linecorp/armeria/internal/common/stream/DecodedStreamMessage.java