Use mockkubeapiserver in metrics_test

[justinsb]

A few changes needed:

• mockkubeapiserver doesn't support patching of maps (yet), as that depends on the OpenAPI, so we switch to ConfigMap and Server-Side-Apply.

• the metrics test relies on watch, so we add a basic implementation of watch to the mockkubeapiserver

[k8s-ci-robot]

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