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

[MrHohn]

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

See #113 (comment) for the context - adding an issue so that we don't lose track.

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.

cc @varunmar @michaelasp