Provide Prometheus metric to count OOM events

[boosty]

We could provide a counter of our PreviousContainerWasOOMKilled events as a Prometheus metric. This would be useful for creating alerts and also for debugging (by seeing how the counter increased over time).

See also the related comments here:

• Log something about OOMKilled containers kubernetes/kubernetes#69676 (comment)
• Update alerting information in README #8 (where @0robustus1 brought up the idea)

[boosty]

For reference, there was an attempt to add such a counter to kubelet, but closed in favour of adding it to the container runtime: kubernetes/kubernetes#87856