Update metrics tracker to most recent specifications

[defagos]

Description

This PR applies changes recently discussed. Specifications themselves are still being worked on but are converging nicely.

Changes made

See related issue for changes.

Checklist

• [x] APIs have been properly documented (if relevant).
• [x] The documentation has been updated (if relevant).
• [x] New unit tests have been written (if relevant).
• [x] The demo has been updated (if relevant).