Fix pagetypeinfo behavior

[fearful-symmetry]

Proposed commit message

closes #32026

So, the linux kernel will truncate page counts over 100k:
https://github.com/torvalds/linux/blob/e5b3efbe1ab1793bb49ae07d56d0973267e65112/mm/vmstat.c#L1521-L1528

We weren't handling this previously, so the metricset would just error out on the string >100000, as it didn't match the provided regex.

This fixes that, and also adds a log line and bit of documentation to clarify how it works.

As the attached issue notes, this is a bit of a hard problem to solve "properly" as attempting to get creative and supplement data from /proc/buddyinfo instead will almost certainly result in misaligned data.

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.

[elasticmachine]

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

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[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 pagetypeinfo-fixes upstream/pagetypeinfo-fixes
git merge upstream/main
git push upstream pagetypeinfo-fixes