Gets the exact number of message backlog for pulsar topic subscription

**Is your feature request related to a problem? Please describe.**
The producer's asynchronous sending interface is batch sending, and a backlog is a batch of messages. To get the number of messages in the subscription backlog, you can only view the number of batches in the backlog, but not the exact number of messages. Expect to support getting the exact number of messages backlog.

Start by tracing how Pulsar currently exposes subscription backlog counts and where batch counts are calculated. Define the intended exact message-count behavior for asynchronous batch sends, then add coverage for the backlog result; done means a subscription reports messages rather than only batches.

Written by the indexing model from the issue text.