feat: use new /metrics endpoint

[lucaspin]

This PR updates the scaler lambda to use the new /metrics endpoint. The endpoint exposes the same numbers as the /occupancy endpoint + the number of agents per state:

{
  "jobs": {
    "queued": 2,
    "running": 8
  },
  "agents": {
    "idle": 12,
    "occupied": 4
  }
}

A new AgentCount metric is pushed to CloudWatch, alongside the current JobCount one:

The /occupancy endpoint is now deprecated and will be removed in the future.