@Override
        public void cancel() {
            assert upstream != null;
            onCancellation(delegate);
            completionFuture.completeExceptionally(CancelledSubscriptionException.get());
            upstream.cancel();
        }
A callback executed when this {@link StreamMessage} is canceled by the {@link Subscriber}.
core/src/main/java/com/linecorp/armeria/common/stream/FilteredStreamMessage.java