feat(metrics): Adding metrics for nfs-provisioner

[mynktl]

Signed-off-by: mayank mayank.patel@mayadata.io

Why is this PR required? What issue does it fix?:

What this PR does?:
This PR exposes metrics about nfs provisioning requests (create/delete). Following metrics are exposed:

persistentvolume_delete_total
persistentvolume_delete_failed_total
persistentvolume_create_total
persistentvolume_create_failed_total

Does this PR require any upgrade changes?: No

If the changes in this PR are manually verified, list down the scenarios covered::

Checklist:

• Fixes #
• PR Title follows the convention of <type>(<scope>): <subject>
• Has the change log section been updated?
• Commit has unit tests
• Commit has integration tests
• (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• (Optional) If documentation changes are required, which issue on https://github.com/openebs/openebs-docs is used to track them:

[mittachaitu]

LGTM

[codecov-commenter]

Codecov Report

Merging #51 (1164d7b) into develop (489da0d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #51   +/-   ##
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  Coverage    48.46%   48.46%           
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  Files           18       18           
  Lines         1595     1595           
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  Hits           773      773           
  Misses         778      778           
  Partials        44       44           

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[kmova]

"Total" here refers to the total number of volumes deleted - by this provisioner pod.

I can think of one usecase for these metrics - to check on the health of the provisioner to see if provisioner is able to successfully create volume or if the failures are increasing.

Are there other usecase for these metrics?

Another interesting metric can be to expose the average duration taken for creation or deletion of the volumes. Probably these can be covered in CSI.

Overall good to merge after getting a response to the usecase query.

[mynktl]

It will help to check stale pv count also from persistentvolume_delete_failed_total