Update Prometheus Client MMAP gem to version that fixes performance degradation

Update prometheus-client-mmap gem to fix performance degradation caused by workers restarting and creating many files with data (one per pid).

See this documentation paragraph for more info https://gitlab.com/gitlab-org/prometheus-client-mmap/tree/master#pid-cardinality

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

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Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
  • 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

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added 1 commit

• eb51ec08 - Update Prometheus Client MMAP gem to version that fixes performance degradation…

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unmarked as a Work In Progress

changed milestone to %9.5

/cc: @bjk-gitlab

changed the description

@DouweM could you review this gem updating MR ?

assigned to @DouweM

LGTM

approved this merge request

merged

mentioned in commit df8ca5aa

Picked into https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14002, will merge into 9-5-stable ready for 9.5.3

mentioned in commit 1cf49129

added devopsmonitor label