Measure bloat of CI job execution

Since a few months we have trace sections which we keep track on gitlab-ce~3098698 for every CI job.

Would it make sense to run a bunch of SQL queries measuring the bloat of each CI job:

1. how long it takes to clone sources?
2. how long it takes to download artifacts?
3. how long it takes to deal with caches?
4. how long it takes to run script?
5. etc.

It could try to answer us how much the bloat is on all preparation phases of CI, and how much further parallelization does help: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23149.

Respective models: Ci::BuildTraceSection.