Performance bar for viewing performance data within GitLab

We should add a "staff" or "profile" mode for anyone with a GitLab instance to see performance data in production, such as SQL timings: https://github.com/blog/1252-how-we-keep-github-fast

I think we used to have something like this in development, but it may have been removed. Perhaps the easiest step would be to enable rack-bug if the users meet some criteria?

Why do we need this?

• It makes it easier for anyone on the team to navigate to specific pages and see why they are slow
• Customers often report slowness, and most of them don't have all the GitLab Performance Monitoring tools set up

Thoughts, @yorickpeterse, @rymai?