Add the system.disk.limit metric

[rogercoll]

Fixes #127

Changes

Adds a new limit metric for system disks.

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://web.archive.org/web/20180321091318/http://www.onlamp.com/pub/a/linux/2000/11/16/LinuxAdmin.html → Status: 503