[PIP-216]: Support subscription backlog metrics in message counts

### Motivation

Now, Pulsar already has a subscription backlog metric, but the unit is entry. And in some scenarios, users want to know the specific number of messages in the subscription backlog.

### Goal

Now, Pulsar already has a subscription backlog metric, but the unit is entry. And in some scenarios, users want to know the specific number of messages in the subscription backlog.

### API Changes

_No response_

### Implementation

.

### Alternatives

_No response_

### Anything else?

_No response_

No file, test, or entry point is identified in the issue. Start by locating the existing subscription backlog metric and determine where its entry-based value is produced; done means exposing the corresponding number of messages in the subscription backlog.

Written by the indexing model from the issue text.