[TRACKING] Transit app banner campaign: metrics to track engagement

[indexing]

We are supporting an MST campaign to promote digital benefits enrollment via Transit app. Before we launch, we want to design and implement metrics to help us understand the effectiveness of this new exploration in promotion.

Issue list

• [x] #2034
• [x] #2035

Acceptance Criteria

• [x] Product can assess effectiveness of the campaign using metrics in Amplitude.
• [x] We can generate multiple unique URLs to associate with specific banners that Transit app will display to MST riders.
• [x] Product can track engagement from each URL:
  • count of visits from each the banner
  • events the user triggered beginning at the app landing page
  • count of successful enrollments originating from banner.

[indexing]

All necessary charts are in place in Amplitude: https://app.amplitude.com/analytics/compiler/chart/lhg1stku/edit/2sv6hvnd