Collector funcs

[campoy]

This PR adds support for new metrics via a new exported package variable CollectorFuncs, following the pattern of the already existing LabelOverrides but adding some thread safety.

It also adds support for extension sources as long as their name has the ext_ prefix.

This is then tested by collecting a hypothetical metric physical_cpu_cores corresponding to half of the virtual cores in requests_quota_hard.

This commit shows the difference in expected results by publishing the new metric.

[k8s-ci-robot]

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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @campoy. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[pwittrock]

/lgtm /approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: campoy, pwittrock

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/usage-metrics-collector/blob/main/OWNERS)~~ [pwittrock] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[pwittrock]

/ok-to-test

[pwittrock]

/lgtm