- The SubscriptionsController is used for SaaS subscriptions and add-ons
  as well. In order to be able to differentiate these in data analytics
  down the line we need to mark when an add-on was purchased and when it
  was a subscription.

- refs:
  https://gitlab.com/gitlab-data/product-analytics/-/issues/158
  https://gitlab.com/gitlab-data/product-analytics/-/issues/158#note_842168023
  https://gitlab.com/gitlab-org/gitlab/-/issues/342853