Unwrap original error from temporary when reporting to operation_errors metric

[eipi-one]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change

/kind bug

/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
When CreateVolume is executed, the csi provisioner expects either "final" or "temporary" errors. If we report final error when filestore instance creation could be ongoing or an instance is already created, then a PVC deletion before successful volume creation could lead to disk leakage as DeleteVolume won't happen.

To prevent filestore instance leakage in FSCSI driver, we changed the error codes returned in CreateVolume to return the Unavailable temporary error code (see #924). As a consequence of this most of the interesting errors we get from CreateVolume, all return Unavailable error code, which is filtered out from SLO.

This PR reports the original error code to the operation_errors metric.

Which issue(s) this PR fixes:

Fixes #925

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

     None

[linux-foundation-easycla]

• ✅login: eipi-one / (f6d8964)

The committers listed above are authorized under a signed CLA.

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[riteshghorse]

LGTM

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: eipi-one, riteshghorse
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[eipi-one]

r: @riteshghorse

[eipi-one]

r: @leiyiz

[eipi-one]

@leiyiz Friendly ping.