[feature][monitoring] Add a metric to track publish buffer usage.

[dao-jun]

Motivation

Add a metric to track broker side publish buffer usage.

Modifications

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

[lhotari]

Since Pulsar is slowly migrating to OTel for metrics, please also handle OTel for this added metric.

For metric changes, it would be useful to at least have a discussion on the dev mailing list since our PIP policy states that a PIP is required for changes in metrics.

[lhotari]

It's not a great idea to add stateful static fields. This causes problems with tests, for example. It also reduces code maintainability.
This should be handled in the BrokerService, I believe.