Add tracking to tier-badge experiment

Refs: Highlight tier in free namespaces at Group/Proj... (#345514 - closed)

Todos

• Add specs to test tracking

What does this MR do and why?

Adds back missing tracking to experiment via snowplow events.

Screenshots or screen recordings

How to set up and validate locally

1. Start GDK with snowplow enabled
2. Enable experiment FF => Rails console: Feature.enable :tier_badge
3. Go to a free group, and click on the CTAs on the badge pop-over
4. Visit http://localhost:9091/micro/good
   1. observe events tracked in log

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.