Add the system.filesystem.limit metric

[rogercoll]

Partially fixes # #127

Changes

Adds a new (opt-in) limit metric for the filesystem.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[rogercoll]

Unrelated CI error:

 ERROR: 1 dead links found in ./docs/system/system-metrics.md !
  [✖] https://blogs.oracle.com/linux/post/understanding-linux-kernel-memory-statistics → Status: 0