Create metric collection

{
  "data": {
    "attributes": {
      "name": "Activation"
    },
    "type": "metric-collections"
  }
}

{
  "data": {
    "attributes": {
      "created_at": "2026-08-01T12:00:00Z",
      "description": "Metrics reviewed for every checkout experiment.",
      "is_guardrail": false,
      "metrics": [
        {
          "metric_id": "550e8400-e29b-41d4-a716-446655440020",
          "metric_name": "Checkout conversion"
        }
      ],
      "name": "Checkout metrics",
      "updated_at": "2026-08-02T09:30:00Z"
    },
    "id": "550e8400-e29b-41d4-a716-446655440040",
    "type": "metric-collections"
  }
}