@Override
        public void onComplete() {
            if (completed) {
                return;
            }
            completed = true;
            try {
                beforeComplete(delegate);
                delegate.onComplete();
                completionFuture.complete(null);
            } catch (Exception cause) {
                delegate.onError(cause);
                completionFuture.completeExceptionally(cause);
            }
        }
A callback executed when this {@link StreamMessage} is canceled by the {@link Subscriber}.
core/src/main/java/com/linecorp/armeria/common/stream/FilteredStreamMessage.java