[improve][client] Client add permits metric

[crossoverJie]

Motivation

Added a new dimensional metric to help better monitor clients.

Modifications

Add pulsar.client.consumer.available_permits.count metric.

Verifying this change

• [x] Make sure that the change passes the CI checks.

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
• [x] The metrics
• [ ] Anything that affects deployment

Documentation

• [ ] doc
• [x] doc-required
• [ ] doc-not-needed
• [ ] doc-complete

Matching PR in forked repository

PR in forked repository: https://github.com/crossoverJie/pulsar/pull/23

[crossoverJie]

@merlimat @asafm PTAL.