Ensure all Prometheus metrics have the same labelnames.

[brian-brazil]

Fixes #1704

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[k8s-ci-robot]

Hi @brian-brazil. 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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[brian-brazil]

I signed it!

[googlebot]

CLAs look good, thanks!

[dashpole]

/ok-to-test

[dashpole]

lgtm, thanks