Expose 2 new metrics about worker health

[mcmarkj]

This exposes 2 new metrics around worker health:

# HELP orchard_worker_last_seen
# TYPE orchard_worker_last_seen gauge
orchard_worker_last_seen{worker_name="Marks-Worker"} 1.725530969e+09

and

# HELP orchard_worker_status
# TYPE orchard_worker_status gauge
orchard_worker_status{status="online",worker_name="Marks-Worker"} 1

That way we can explicitly alert if a node goes offline.

[CLAassistant]

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[mcmarkj]

While I'm here, should orchard_vms be constantly incrementing on each scrape? Would it make sense to change that from this to something that just sets it to a count?

    for _, vm := range vms {
        vmsStat.With(map[string]string{"status": string(vm.Status)}).Inc()
    }