Add retention metrics and CTR metric for image browsing

Jenniferwangrequested to merge
jiawang/experiment-analytics-configs:we3_1-image-browsing_ab_test into main

Metrics include: 8 Retention metrics for mobile web:

  • 2 versions of 2nd day retention rate: 1) (n-2)-day cohort window 2) 7-day cohort window
  • 2 versions of 2nd week retention rate: 1) (n-14)-day cohort window 2) 7-day cohort window
  • 2 versions of 2 continued return on 2nd and 3rd days: 1) (n-3)-day cohort window 2) 7-day cohort window
  • 2 versions of 21 day cumulative retention rate: 1) 14-day cohort window 2) 7-day cohort window
    1 CTR metric for feature: clickthroughs rate per user

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