metric/system: preallocate arrays

[florianl]

What does this PR do?

Whenever possible, preallocate array to the correct size instead of letting arrays "grow". This reduces the load on the Garbage Collector and improves memory usage efficiency.

Checklist

• [x] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.md

[florianl]

Linux, Windows and MacOS CI builds: https://buildkite.com/elastic/elastic-agent-system-metrics/builds/125