Bump prometheus-client-mmap to v0.21.0

What does this MR do and why?

This commit brings in the Rust implementation of to_metrics but does not yet use it.

This version also shells out to getconf PAGESIZE once to determine the page size for the operating system.

v0.21.0 requires RubyGems >= v3.3.22 since there is a bug in older versions that may install the musl version of the precompiled native extensions: https://github.com/rubygems/rubygems/pull/5852.

Diff: https://my.diffend.io/gems/prometheus-client-mmap/0.19.1/0.21.0

Relates to #408338 (closed)

Tests

These are the tests that I ran for Omnibus GitLab:

1. Confirmed curl http://localhost:8080/-/metrics continues to work.
2. For x86 platforms, confirmed the x86_64-linux precompiled, native gem is installed:

    root@stanhu-gce:/opt/gitlab/embedded/bin# ./gem list | grep prometheus-client-mmap
    prometheus-client-mmap (0.21.0 x86_64-linux)

3. Confirmed Rust extension is also available via gitlab-rails console:

   irb(main):001:0> Prometheus::Client::Formats::Text.rust_impl_available?
   => true

• Ubuntu 20.04 package build: https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/4186217693. ✅
• CentOS 7 package build: https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/4186225967 ✅ (this verifies that the older glibc works fine with the precompiled extension)
• AmazonLinux 2 arm64: https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/4186225967 ✅ (this verifies that the forced recompile for arm64 works, since rake-docker-compile uses Ubuntu 20.04 as the base image).

  [ec2-user@ec2-instance]$ ./gem list prometheus-client-mmap
  
  *** LOCAL GEMS ***
  
  prometheus-client-mmap (0.21.0)

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.