chore(metrics): fix metric reader types and tests

[anuraaga]

Description

While implementing metric reader metrics, I ran into some issues with the test because currently the tests seem incorrect, along with some of the type annotations. The metric pipeline uses MetricsData but certain types and mocks were using list[Metric]. This corrects the types and aligns tests with them - tests previously passed due to duck typing despite not really being valid usage

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Unit tests

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[xrmx]

@anuraaga please add a changelog entry, it's good to have trace of typing change even if it is for internal interfaces