Tune Ruby GC for gitlab-exporter

What does this MR do?

This is the last round of tuning to make gitlab-exporter more memory efficient. Here we tune the Ruby GC to better match gitlab-exporter work loads.

Related issues

gitlab-org/gitlab#297241 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• [-] Test plan indicating conditions for success has been posted and passes. See the referenced issue for how we tested this.
• [-] Documentation created/updated
• [-] Tests added
• [-] Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened: gitlab-org/omnibus-gitlab!4987 (merged)