Measure # of unique users that are doing commit for .gitlab-ci.yml on gitlab.com projects (GMAU on .com)

This is a follow up from #287683 (comment 466108398)

Similar to #287683 (closed) we would like to track the number of unique users that are doing commit for .gitlab-ci.yml

This would give us the true number of users that are interacting with .gitlab-ci.yml file

Proposal

Look into instrumenting with Snowplow / Redis if this data is not readily available.