chore: add labels to metrics

[marcelocyreno]

What type of PR is this?
/kind feature
/kind documentation

What this PR does / why we need it:

• Adds additional labels pod_name, pod_namespace and secret_provider_class to the existing metrics.
• Fixes documentation about current metrics

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #1344

Special notes for your reviewer:
This PR contains the minor refinements suggested in the review of #1433.
Original work credit goes to @Suraiya-Hameed.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

[k8s-ci-robot]

Hi @marcelocyreno. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: marcelocyreno
Once this PR has been reviewed and has the lgtm label, please assign ritazh for approval. For more information see the Kubernetes Code Review Process.

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[hakman]

/ok-to-test
/cc @nilekhc

[marcelocyreno]

/retest

[hakman]

/test pull-secrets-store-csi-driver-image-scan

[enj]

/hold

pod_name, pod_namespace and secret_provider_class are all unbounded cardinality which is not appropriate for metrics.