Update filesystem and fsstat docs to update the definition of a "file"

[fearful-symmetry]

What does this PR do?

I recently realized that what the system/fsstat and system/filesystem calls files are in fact inodes. A lot of users might not even notice the difference, as the total count of files and used inodes can be fairly similar, but I figured it was at least worth updating the docs for anything that decides to dig deeper. It would be nice to update the fields themselves to be *.inodes where appropriate, but we can't really do that without breaking existing mappings, and aliasing the fields seems confusing in its own right, as files and inodes are conceptually rather different.

Why is it important?

This is a doc change that more accurately describes what some of the fields actually mean.

Checklist

• I have made corresponding changes to the documentation
• 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. Could you fix it @fearful-symmetry? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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[cmacknz]

The other option here would be to have the metrics actually report the number of files instead of inodes, and add new metrics for inodes.

I don't think this affects correcting the documentation, but should we have an issue to consider doing that?

[fearful-symmetry]

@cmacknz so, there's no good, OS-independent, filesystem-independent way to get the number of files. Doing something equivalent find -type f can be super slow and potentially hammer metricbeat and/or the filesystem. We could potentially derive something, but I'm not sure how much more accurate that would be than just telling users that the inode count is probably close.

[cmacknz]

@cmacknz so, there's no good, OS-independent, filesystem-independent way to get the number of files. Doing something equivalent find -type f can be super slow and potentially hammer metricbeat and/or the filesystem. We could potentially derive something, but I'm not sure how much more accurate that would be than just telling users that the inode count is probably close.

Thanks, that makes sense. If you are updating the code comments can you add this explanation there as well? That will save the next person trying to understand this some time.

[v1v]

@fearful-symmetry , I'm afraid the backport-v8.2.0 is not yet available, and likely it points to main since main is 8.2 based. What branch would you need to backport this particular PR to? The backport definitions can be found in https://github.com/elastic/beats/blob/main/.mergify.yml#L210-L248

[fearful-symmetry]

@v1v so, I actually assumed it would be in 8.2, I think I just keep adding the backport label out of habit, since that's a little closer to how the backports worked pre 8.0.