For M54 we have some UMA for number of attempts so that may give us some idea of requests that are dropped after 4 attempts but won't be able to differentiate from requests that finally succeeded on their 4th attempt.

Be good to have UMA for these dropped request cases:
  - too many completed attempts (failures)
  - too many started attempts - where prerender calls back
  - too many started attempts - garbage collected by Picker (may be crash loop or process kill loop)
  - expired requests

But perhaps any Remove cause should be included as well - eg, user-requested Removes.