Fix bug causing truncation of Metrics files

Bump Prometheus Client gem version that fixed a bug with truncating metrics file.

Why was this MR needed?

These changes in https://gitlab.com/gitlab-org/prometheus-client-mmap/merge_requests/4 Are in the newest version of the Gem

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related https://gitlab.com/gitlab-org/gitlab-ee/issues/2993