Update prometheus-client-mmap to v0.17.0

What does this MR do and why?

This fixes a seg fault when strings stored in ObjectSpace are inspected: gitlab-org/ruby/gems/prometheus-client-mmap!74 (merged)

Relates to gitlab-org/ruby/gems/prometheus-client-mmap#39 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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

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