@Override
    public Set<String> subscription() {
        acquireAndEnsureOpen();
        try {
            return Collections.unmodifiableSet(subscriptions.subscription());
        } finally {
            release();
        }
    }
Get the current subscription.  or an empty set if no such call has
been made.
@return The set of topics currently subscribed to
clients/src/main/java/org/apache/kafka/clients/consumer/internals/AsyncKafkaConsumer.java