Update dependency gitlab-exporter to v15.2.0

What does this MR do and why?

This backports !8147 (merged) to 17-8-stable.

This makes gitlab-exporter able to work with Prometheus v3: gitlab-org/ruby/gems/gitlab-exporter#108 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).