Update metrics tracker to most recent specifications

[defagos]

Description

This PR applies changes recently discussed. Specifications themselves should now be stable or almost stable. This should therefore be one of the latest associated PRs.

Changes made

• See related issue for changes related to the specifications.
• Improved implementation by building common payload at a single location.
• Always use event creation date as timestamp.
• Implement proper session management.

Checklist

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

[defagos]

@waliid Thanks for the review!