curl --request GET \
  --url https://core.nozle.app/api/v1/billable_metrics/{code} \
  --header 'Authorization: Bearer <token>'

{
  "billable_metric": {
    "lago_id": "1a901a90-1a90-1a90-1a90-1a901a901a90",
    "name": "Storage",
    "code": "storage",
    "recurring": false,
    "created_at": "2022-09-14T16:35:31Z",
    "aggregation_type": "sum_agg",
    "description": "GB of storage used in my application",
    "rounding_function": "round",
    "rounding_precision": 2,
    "expression": "round((ended_at - started_at) * units)",
    "field_name": "gb",
    "weighted_interval": "seconds",
    "filters": [
      {
        "key": "region",
        "values": [
          "us-east-1"
        ]
      }
    ]
  }
}