[improve][broker] add namespace topics limit metric

[alexandrebrg]

Motivation

We recently came with an issue where namespaces were reaching their limits, errors are lifted to producers and cannot be fixed by them (need either to delete topics or increase namespace limit). This PR creates a new metric to give topics limit for each namespace loaded in a broker. It will allow to correlate the current topic count (already existing metric) and the limit to determine the saturation of a namespace.

Modifications

• Implement metric pulsar_namespace_max_topics_limit (labels are cluster and namespace)

Verifying this change

• Make sure that the change passes the CI checks.

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

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

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[github-actions]

@alexandrebrg Please add the following content to your PR description and select a checkbox:

- [ ] `doc` <!-- Your PR contains doc changes -->
- [ ] `doc-required` <!-- Your PR changes impact docs and you will update later -->
- [ ] `doc-not-needed` <!-- Your PR changes do not impact docs -->
- [ ] `doc-complete` <!-- Docs have been already added -->

[dao-jun]

@alexandrebrg Thanks for your contribution!
But a metrics change needs a PIP, you can check the PIP progress there: https://github.com/apache/pulsar/blob/master/pip/README.md

By the way, could please add tests for the change?