Introduce disk/percent_used metric, refs #822

[AndreMiras]

The Stackdriver exports the /guest/disk/percent_used metric to the custom.googleapis.com namespace as the reserved one compute.googleapis.com cannot be used at this stage.

This was tested within GCP Container-Optimized OS with the following:

/mnt/disks/scratch/node-problem-detector \
--enable-k8s-exporter=false \
--config.system-stats-monitor=/etc/node_problem_detector/system-stats-monitor.json \
--config.system-log-monitor=/etc/node_problem_detector/kernel-monitor.json \
--config.custom-plugin-monitor=/etc/node_problem_detector/boot-disk-size-consistency-monitor.json \
--exporter.stackdriver=/etc/node_problem_detector/stackdriver-exporter.json

The /mnt/disks/scratch/ directory was mounted specifically to get execution permissions:

sudo mount -t tmpfs tmpfs /mnt/disks/scratch/

This is what it looks like in GCP Monitoring:

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: AndreMiras / name: Andre Miras (09a5d48)

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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PR needs rebase.

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[AndreMiras]

superseded by #825