Cherry-pick #27374 to 7.x: Add formatting to pressure fields in system/process

[fearful-symmetry]

Cherry-pick of PR #27374 to 7.x branch. Original message:

What does this PR do?

I forgot to add field formatting directives while adding cgroupsV2 support. This adds them to *.pressure.*.pct.

Why is it important?

The formatting directives should be there.

Checklist

• My code follows the style guidelines of this project
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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• Start Time: 2021-08-17T17:43:01.745+0000

• Duration: 96 min 17 sec

• Commit: d1a76cd

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Test	Results
Failed	0
Passed	9232
Skipped	2336
Total	11568

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Test stats 🧪

Test	Results
Failed	0
Passed	9232
Skipped	2336
Total	11568

[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 backport_27374_7.x upstream/backport_27374_7.x
git merge upstream/7.x
git push upstream backport_27374_7.x