mod: update version of elastic-agent-system-metrics to v0.4.6

[efd6]

What does this PR do?

This updates the version of elastic-agent-system-metrics to v0.4.6.

Why is it important?

A bug in previous versions of the library prevents builds on mips family GOARCHs.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

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[elasticmachine]

💚 Build Succeeded

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• Start Time: 2023-02-27T22:41:03.645+0000

• Duration: 102 min 33 sec

Test stats 🧪

Test	Results
Failed	0
Passed	25979
Skipped	1962
Total	27941

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Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mips-rdev upstream/mips-rdev
git merge upstream/main
git push upstream mips-rdev