[Refactor] extract metrics to a single source of truth

[rosekamallove]

• [x] Create a draft
• [x] Change the metric object into a single Data Structure (Should contain everything except validation code)
• [x] Implement the Data Structure
• [ ] Write a test for validation

[rosekamallove]

Hey, can you please review it?

[rosekamallove]

A small question, should I push the test in the PR or create a new one for it?

[ljharb]

Refactors don't generally need tests; if you have a unit test in mind for the Metrics file that's testing structure but not hardcoding expected metric contents, then it'd be great to add that here.

[ljharb]

Actually I'm going to land as-is; we can add the test later if you're inclined.

[rosekamallove]

We can talk about this in detail tomorrow at the meeting.