metrics: unit testing strategy

[alan-churley]

Description of the feature request

Problem statement Metrics unit tests currently are not structured the same as the Logger module,

In the logger only public methods are tested, following the recommendations here: https://github.com/goldbergyoni/javascript-testing-best-practices/#-%EF%B8%8F-14-stick-to-black-box-testing-test-only-public-methods

Opening a ticket to track discussions and work on improving this,

Related issues, RFCs

https://github.com/awslabs/aws-lambda-powertools-typescript/pull/102