agent: add ExtraIP per-VM metric

[Omrigan]

Plus a couple of refactorings.

To be merged with rebase.

Part of #1294.

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[sharnoff]

Refactorings seem mostly fair enough, one question.

re: Adding the metric — I wonder if we actually need this? What could you do with such a metric that you couldn't do before? (I'm imagining that you can always cross-reference with cplane's view of the VM if you want to check whether it has the overlay network enabled ­— but maybe I'm missing something!)

[Omrigan]

Adding the metric — I wonder if we actually need this? What could you do with such a metric that you couldn't do before?

I thought of two use cases:

1. View it in "per-VM metrics" dashboard, if we need to debug computes and network around it.
2. Aggregate it to see the share of computes in the overlay network.

(I'm imagining that you can always cross-reference with cplane's view of the VM if you want to check whether it has the overlay network enabled ­— but maybe I'm missing something!)

Yeah, that's fair, but cplane info is not in grafana.

[sharnoff]

Looks good. A couple questions