Add Ceph usage by device class metric

[apritcha1]

What does this PR do?

Adds a metric for the percentage of storage used in Ceph, broken down by device class.

Motivation

The Ceph integration does not currently report the amount of space utilized by device class, leading to a lack of visibility over potential issues with a specific device class becoming full. According to the documentation the ceph df command has provided a RAW STORAGE section (called stats_by_class in the JSON output) since Nautilus, enabling this reporting across the six newest versions of Ceph.

Additional Notes

I have not updated the unit test for the new metric, as I am unsure if there are any special requirements related to generating/updating the raw.json/raw2.json fixtures required. I apologize if I have missed any documentation on this topic. Please let me know what is needed to satisfy any requirements I am missing.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged