Make metrics label Snake case compliant

[skmatti]

prometheus-to-sd requires the labels to named in snake case format.

[k8s-ci-robot]

Hi @skmatti. Thanks for your PR.

I'm waiting for a kubernetes 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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[skmatti]

/assign @freehan

[rramkumar1]

/ok-to-test

[bowei]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bowei, skmatti

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