Bump gitlab-exporter to v15.2.0

What does this MR do?

This backports !2207 (merged) to 17-8-stable in order to provide compatibility with Prometheus v3.

Related issues

• gitlab-org/ruby/gems/gitlab-exporter#108 (closed)
• https://github.com/prometheus/prometheus/issues/15777
• !2233 (merged)

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
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated
• New dependencies are managed with GitLab forked renovatebot