[RPD-249] Refactor `analytics_service` to simplify tracking decorator

[swells2020]

This PR refactors the analytics_service module by breaking the functionality of the main decorator function out into a small number of private utility functions. Each function is then independently tested.

Checklist

Please ensure you have done the following:

• [x] I have read the CONTRIBUTING guide.
• [ ] I have updated the documentation if required.
• [x] I have added tests which cover my changes.

Type of change

Tick all those that apply:

• [ ] Bug Fix (non-breaking change, fixing an issue)
• [ ] New feature (non-breaking change to add functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Other (add details above)