Use Runner UUID as subject in snowplow billing event
What does this MR do?
Use Runner UUID as subject in snowplow billing event
As per gitlab#594605 (closed), we should use the newly created UUID that's tracked for each Runner enity on GitLab side as the subject value for the billing event setnt to Snowplow. This will allow us to align all three parts (GitLab, GitLab Runner, Billing Platform) with a globally unque UUID representing a given runner.