Spy on feature flags 🕵️

[mariusandra]

Changes

• Use a Proxy to spy on the feature flags object.
• Send a $feature_flag_called event on first access.
• Fallback to getters/setters for IE11 or other rare cases without Proxy
• Helps with Bug with $feature_flag_called metrics #2614 (comment)

WIP because I haven't run the code yet. My dev setup is under (thanks big sur). Will fix and test.

Checklist

• All querysets/queries filter by Organization, by Team, and by User
• Django backend tests
• Jest frontend tests
• Cypress end-to-end tests

[mariusandra]

Seems to work.
If needed we can change the granularity of events sent (once per visit vs once per minute or whatever time period).

[paolodamico]

love the JS magic! code looks good and I tested on the staging env and everything seems to work as expected 💪. Perhaps my only comment is that it'd be great to add some tests here

[timgl]

Why aren't we just using the native posthog.isFeatureEnabled instead of featureFlagLogic?

[mariusandra]

If we put this inside a react render component, then in my tests, the action and navigation feature flags were checked 6 times per render and hundreds of times when browsing around. That's too many events.