Add volume heal entries count metric

[ricmatsui]

What does this PR do?

Adds a new metric for the GlusterFS integration to monitor the number of entries requiring healing in a volume by parsing additional output from gstatus.

• https://docs.gluster.org/en/main/Administrator-Guide/Managing-Volumes/#triggering-self-heal-on-replicate
• https://github.com/gluster/gstatus/blob/b3abf1b7169b5db5e228716a1a2fec2c07c9ff65/gstatus/glusterlib/cluster.py#L183
• https://github.com/gluster/glustercli-python/blob/920ddcec3d774de28a7f8cf04e033f0d0fa40c13/glustercli/cli/parsers.py#L724
• https://github.com/gluster/glusterfs/blob/8641282a759ed057a386bbb6d145511302bf6a1a/heal/src/glfs-heal.c#L232

Motivation

Currently the integration is missing a way to monitor if replicated volumes are failing to self-heal.

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

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.99%. Comparing base (026cb75) to head (d8904d5).
⚠️ Report is 16 commits behind head on master.

Additional details and impacted files

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