Cant ack messages on exclusive subscriptions with partioned topic

[st0ckface]

Describe the bug
When creating a consumer with an exclusive subscription, acked messages are not reflected in broker stats.

To Reproduce
Steps to reproduce the behavior:

1. Create a consumer with an exclusive subscription to a partitioned topic
2. Ack each message individually as they are received
3. Notice that "unackedMessages" continually climbs in the topic stats via the admin client

static PulsarClient getJPulsarClient() throws Exception{
        return PulsarClient.builder()
                .serviceUrl("pulsar://localhost:6650")
                .connectionsPerBroker(20)
                .ioThreads(8)
                .listenerThreads(8)
                .build();

    }

    static Consumer<byte[]> getJPulsarConsumer(PulsarClient client, String topic, String subscription) throws Exception {
      return  client.newConsumer(Schema.BYTES)
                .subscriptionName(subscription)
                .subscriptionType(SubscriptionType.Exclusive)
                .topic(topic)
                .consumerName("ackTestConsumer")
              .subscribe();
    }

    public static void main(String... args) throws Exception{
        PulsarClient client = getJPulsarClient();
        String topic = "public/default/870|EXTERNID_NG|2020-03-17";
        String subscription = "ackTestSubscription"
        Consumer<byte[]> consumer = getJPulsarConsumer(client, topic, subscription);

        int maxMessages = 1000000;
        List<CompletableFuture<Object>> futures = new ArrayList<>();
        for(int i = 0; i < maxMessages; i ++){
            futures.add(consumer.receiveAsync().thenApplyAsync(m -> {
                try {
                    consumer.acknowledge(m.getMessageId());
                } catch (PulsarClientException e) {
                    e.printStackTrace();
                }
                return null;
            }));
        }

        CompletableFuture.allOf(futures.toArray(new CompletableFuture[futures.size()])).get();
    }

Expected behavior
The "unackedMessages" metric should at most hover at or around the receiveQueueSize if not 0. The backlog should also be descreasing

Screenshots
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Desktop (please complete the following information):

• OS: N/A

Additional context
If the subscription is changed to Shared, the topicStats produce the expected behavior (showing messages acked)