Review `Gitlab::Profiler` and take action

https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/profiler.rb

1. Does it work? Yes, as long as the rubyprof call does not time out
2. Is it used? Yes, according to the survey, 3 (out of 17) people use(d) Gitlab::Profiler (before)
3. Is it helpful? Yes, profiling a page is helpful. Ruby-prof might not work great with ruby 3, and it times out on huge pages. So we might look into that, perhaps switch to stackprof?
4. It is easy to use? Yes, the bin/profile-url script makes this very easy.

What does GitLab::Profiler do and (how) do we use it?

You can call GitLab::Profiler.profile with a URL. It will then run RubyProf.profile on that URL and return the RubyProf profiling report (as HTML)

There is also GitLab::Profiler.with_user, which wraps the block with a logged in user

It can easily be used by running the bin/profile-url script like so:

bin/profile-url flightjs/Flight/-/merge_requests/4/diffs --user=root --output=/tmp/profile.html --sql=/tmp/profile_sql.txt

A lot of info can be found on the docs:

Docs: https://docs.gitlab.com/ee/development/profiling.html

https://docs.gitlab.com/ee/administration/troubleshooting/gitlab_rails_cheat_sheet.html#profile-a-page

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In short: The Gitlab::Profiler looks very useful for local profiling. The only possible issue is that it uses 'rubyprof', but we might have to switch that to stackprof when we switch to Ruby 3.0