Track usage of Visual Review Tools

Problem to solve

We cannot measure that we have met the outcomes we wanted with https://gitlab.com/gitlab-org/gitlab-ee/issues/10761 when we added the ability to make comments on the merge request associated with a Review App directly from within the review app.

Proposal

This issue should add statistics around this feature's usage. Specific things we will be interested in is:

• How many times has the Review App has been opened
• How many times has a comment been pushed back to an MR

Other considerations

• We are going to try to use snowplow for this but have no experience with using that platform yet
• Another issue in this milestone gitlab-#29067 is removing authentication for the visual review tools so we cannot guarantee we would have unique userids opening / commenting on a visual review tool

Items previously discussed but not tracking at this time

• Users that include the Visual Review Tool code in their app
• Unique users that add at least 1 discussion item from Visual Review Tool

@erushton @jhampton can you weigh in on which of these could be tracked today?