@Override
    public void subscribe(Subscriber<? super HttpData> subscriber, EventExecutor executor,
                          SubscriptionOption... options) {
        requireNonNull(subscriber, "subscriber");
        requireNonNull(executor, "executor");
        requireNonNull(options, "options");
        final MultipartEncoder encoder = new MultipartEncoder(parts, boundary);
        encoder.subscribe(subscriber, executor, options);
    }
Returns a random boundary used for encoding multipart messages.
core/src/main/java/com/linecorp/armeria/common/multipart/DefaultMultipart.java