List billable metrics

curl --request GET \
  --url https://api.alguna.io/metrics \
  --header 'Alguna-Version: <alguna-version>' \
  --header 'Authorization: Bearer <token>'

{
  "data": [
    {
      "aggregation": {
        "field": "amount",
        "method": "sum"
      },
      "created_at": "2026-04-01T10:00:00Z",
      "event_name": "api_call",
      "id": "mtr_01H1VECT",
      "name": "API Calls",
      "updated_at": "2026-04-01T12:30:00Z",
      "description": "Counts API calls per customer",
      "filter_groups": [
        {
          "filters": [
            {
              "field": "region",
              "operator": "equal",
              "value": "us-east-1"
            }
          ],
          "operator": "and"
        }
      ],
      "tag_ids": [
        "<string>"
      ]
    }
  ],
  "pagination": {
    "per_page": 20,
    "total_pages": 5
  }
}