refactor(metrics): unit tests for Metrics are written to follow similar convention as Logger/Tracer

[arnabrahman]

Description of your changes

Metrics unit tests were not following the same convention as Logger/Tracer package. In Logger/Tracer tests, only public functions are unit tested. But for Metrics, unit tests are written per feature/behavior.

This PR updates the tests to match the same convention as Logger/Tracer package. Only public methods are tested, following the recommendations here: Best Practices

100% test coverage is maintained. Some additional housekeeping is implemented, constants & helpers are introduced for better code maintainability.

I've removed the previous feature tests because in my understanding leaving them will create further confusion about the testing patterns. I can add them again if maintainers feel the current sets of tests in this PR are inadequate.

How to verify this change

All the tests will run successfully along with 100% test coverage.

Related issues, RFCs

Issue number: #163

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published
• The PR title follows the conventional commit semantics

Breaking change checklist

Is it a breaking change?: NO

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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[dreamorosi]

Hi @arnabrahman thank you so much for opening this PR!

Given the size, please allow us some time to go through it and review it.

I'll get back to you here with comments as I review it.

[dreamorosi]

I've done a first review and things are looking good!

I have left one comment about the helper function, that I think we could drop.

Now I'm going to focus on reviewing the actual test cases and see if all the previous ones are covered.

[dreamorosi]

Hi @arnabrahman thank you so much for the work you've done here.

I really appreciate the effort and dedication you've put into improving the project.

[arnabrahman]

Happy to help