Expose OOM event count to prometheus

[kragniz]

Fixes #1837.

[k8s-ci-robot]

Hi @kragniz. Thanks for your PR.

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[kragniz]

I'm not convinced this is the best way to get the event data through, but seemed like the most straightforward. If this is an acceptable method, I can add tests and tidy this up further.

[kragniz]

/assign @bobbypage

[kragniz]

@iwankgb thanks for the reviews, I've updated with fixes

[iwankgb]

/ok-to-test

[kragniz]

/retest

[darkl0rd]

This metric (Docker) always returns 0 for me - see also #3015