add host network metrics

[wtip]

I need some host network metrics so that I can keep an eye on host level network bottlenecks. I probably would like to see throughput for each nic and not just an aggregate host throughput which seems useless. I was trying to figure out what the net.throughput.contention.summation metric actually reports as this might be an easier alternative.

[pryorda]

This issue was moved to pryorda/vmware_exporter#12