Bump prometheus-client-mmap to v0.20.X with Rust support

• Merged in !117631 (merged)
• Reverted in !118325 (merged)

The following discussion from !117631 (merged) should be addressed:

• @splattael started a discussion: (+1 comment)

  Reverting due to 🔴 https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/4154544371#L1130

  LoadError: cannot load such file -- fast_mmaped_file
  /var/cache/omnibus/src/gitlab-rails/config/application.rb:15:in `<top (required)>'
  /var/cache/omnibus/src/gitlab-rails/Rakefile:9:in `require'
  /var/cache/omnibus/src/gitlab-rails/Rakefile:9:in `<top (required)>'
  /opt/gitlab/embedded/bin/bundle:23:in `load'
  /opt/gitlab/embedded/bin/bundle:23:in `<main>'
  Caused by:
  LoadError: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.33' not found (required by /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/prometheus-client-mmap-0.20.3-x86_64-linux-musl/lib/3.0/fast_mmaped_file.so) - /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/prometheus-client-mmap-0.20.3-x86_64-linux-musl/lib/3.0/fast_mmaped_file.so
  /var/cache/omnibus/src/gitlab-rails/config/application.rb:15:in `<top (required)>'
  /var/cache/omnibus/src/gitlab-rails/Rakefile:9:in `require'
  /var/cache/omnibus/src/gitlab-rails/Rakefile:9:in `<top (required)>'
  /opt/gitlab/embedded/bin/bundle:23:in `load'
  /opt/gitlab/embedded/bin/bundle:23:in `<main>'
  (See full trace by running task with --trace)

  Edit: See !118325 (merged)