Update prometheus gem to version 0.9.1.pre.rc.2 fixing segfaults happening on metrics deletion

What does this MR do?

It update prometheus gem to version that include fixes from https://gitlab.com/gitlab-org/prometheus-client-mmap/merge_requests/19.

That version is able to detect and avoid states where datastructures linked to metric files were unmmapped causing segmentation faults.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

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