Add node "status", "scheduling eligibility" to all client metrics

[pete-woods]

• We previously added these to the client host metrics, but it's useful to have them on all client metrics.
• e.g. so you can exclude draining nodes from charts showing how much memory / CPU capacity your fleet has.

[pete-woods]

It would be awesome if we could backport this to the 12.x series, too, as this is kinda a bugfix, too (makes your client metrics look really weird if you start filtering by the ready and eligability tags).

[pete-woods]

Apologies for the ping @langmartin and @schmichael, but you've reviewed my PRs in this area before, and was hoping for a quick look over this one! :)

[tgross]

Hi @pete-woods! Thanks for this PR!

LGTM. I built this PR and smoke-tested it with the default example job. Then I can query the metrics and get things like the following:

$ curl -s http://localhost:4646/v1/metrics | jq '.Gauges[] | select(.Name == "nomad.client.allocated.memory")'
{
  "Labels": {
    "node_scheduling_eligibility": "eligible",
    "host": "linux",
    "node_id": "3dc8f04b-9158-8045-ba03-c41748ef8a5d",
    "datacenter": "dc1",
    "node_class": "none",
    "node_status": "ready"
  },
  "Name": "nomad.client.allocated.memory",
  "Value": 256
}

Just a heads up @pete-woods that Lang has moved on to a new project, but the whole team is always happy to review PRs!

[pete-woods]

Thanks for merging, @tgross! Is it in any way possible to backport this fix to the 0.12 series, or is that reserved for really serious stuff?

[tgross]

Hi @pete-woods. Our intent at this point is not to ship another 0.12.x series unless there's a critical bug or security flaw.

[pete-woods]

Fair enough! I shall await 0.13 then.

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