buildkite-job-dispatcher: replace graphql request with https://agent.buildkite.com/v3/metrics

Created by: bobheadxi

TIL about this endpoint https://buildkite.com/docs/apis/agent-api/metrics which possibly gives us what we need for buildkite-job-dispatcher to make scaling decisions, in a nice condensed format, while bypassing the buggy and unreliable GraphQL API

curl -H "Authorization: Token $TOKEN" "https://agent.buildkite.com/v3/metrics" | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   408    0   408    0     0    791      0 --:--:-- --:--:-- --:--:--   804
{
  "agents": {
    "idle": 36,
    "busy": 1,
    "total": 37,
    "queues": {
      "baremetal": {
        "idle": 1,
        "busy": 0,
        "total": 1
      },
      "stateless": {
        "idle": 25,
        "busy": 1,
        "total": 26
      },
      "vagrant": {
        "idle": 10,
        "busy": 0,
        "total": 10
      }
    }
  },
  "jobs": {
    "scheduled": 0,
    "running": 2,
    "waiting": 4,
    "total": 6,
    "queues": {
      "default": {
        "scheduled": 0,
        "running": 0,
        "waiting": 4,
        "total": 4
      },
      "standard": {
        "scheduled": 0,
        "running": 2,
        "waiting": 0,
        "total": 2
      }
    }
  },
  "organization": {
    "slug": "sourcegraph"
  }
}