extract pod representation from backend/metrics to backend

[nirrozenbaum]

This PR extracts the Pod minimal representation from backend/metrics to backend.
I'm working on other PRs that implement scraper concept to allow extensibility for pod scraping.
for the use of this, it's useful to reuse the same pod definition.

since this change touches a lot of files that are using the pod definition, it makes sense to push it in a separate PR.

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[nirrozenbaum]

CC @kfswain @ahg-g

[kfswain]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kfswain, nirrozenbaum

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[kfswain]

/lgtm