Always collect metrics for the root ns and optionally collect metrics for the pod namespaces

Basically the way we collect netlink metrics makes it necessary to turn on `hostPID`, which we are not ready to do yet.

See https://github.com/GoogleCloudPlatform/netd/pull/113#discussion_r692547470 for the context - adding an issue so that we don't lose track.
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I think we can do two things - always collect metrics for the root ns (and label it by the node IP I guess) and optionally if the pod watch is enabled, also collect metrics for the pod namespaces.

I think this will require some additional work to figure out how to export the root ns metrics into a K8sContainer schema. So for now, I think just create an issue so we don't lose track and we can merge this PR.
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cc @varunmar @michaelasp