[improve][broker] Implemented ExtensibleLoadManagerWrapper.getLoadBalancingMetrics()

[heesung-sn]

Master Issue: apache#16691

Motivation

We will start raising PRs to implement PIP-192, apache#16691

Modifications

This PR implemented ExtensibleLoadManagerWrapper.getLoadBalancingMetrics()and its unit test.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• *Added unit tests.

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
• Anything that affects deployment

Documentation

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

We will have separate PRs to update the Doc later.

Matching PR in forked repository

PR in forked repository: #28