feat(invoice): feature cost

[hekike]

This is a PoC to help understand the cost behind invoice line items and break them down by meter dimensions.
This is useful to understand the invoiced cost of individual users, workspaces, etc.

TODO

• Finalize API
• Move into service
• Add tests

Invoice Feature Cost

GET /api/v1/billing/invoices/01K3M32GV47W7RD1XR93091X2Y/lines/01K3M32GVCEZ8BTG62DX0NVBQH/cost?windowSize=MINUTE&groupBy=model

{
  "costPerUnit": "0.01",
  "currency": "USD",
  "from": "2025-08-26T21:03:19Z",
  "rows": [
    {
      "cost": "1",
      "groupBy": {
        "model": "gpt-3"
      },
      "usage": "100",
      "windowEnd": "2025-08-26T21:10:00Z",
      "windowStart": "2025-08-26T21:09:00Z"
    },
    {
      "cost": "1",
      "groupBy": {
        "model": "gpt-3"
      },
      "usage": "100",
      "windowEnd": "2025-08-26T21:15:00Z",
      "windowStart": "2025-08-26T21:14:00Z"
    },
    {
      "cost": "1",
      "groupBy": {
        "model": "gpt-4"
      },
      "usage": "100",
      "windowEnd": "2025-08-26T21:15:00Z",
      "windowStart": "2025-08-26T21:14:00Z"
    }
  ],
  "to": "2025-09-26T21:03:19Z",
  "totalCost": "3",
  "totalUsage": "300"
}

Invoice

GET /api/v1/billing/invoices/01K3M32GV47W7RD1XR93091X2Y

{
  "collectionAt": "2025-09-26T21:03:19Z",
  "createdAt": "2025-08-26T21:03:19.396259Z",
  "currency": "USD",
  "customer": {
    "id": "01K3M32BS0MCA6ZCZQS60EA5CT",
    "name": "Customer 2",
    "usageAttribution": {
      "subjectKeys": [
        "customer-2"
      ]
    }
  },
  "id": "01K3M32GV47W7RD1XR93091X2Y",
  "lines": [
    {
      "children": [
        {
          "category": "regular",
          "createdAt": "2025-08-26T14:20:30.396154-07:00",
          "currency": "USD",
          "id": "01K3M41ZNW07N1NZM67DF4DZT8",
          "invoice": {
            "id": "01K3M32GV47W7RD1XR93091X2Y"
          },
          "invoiceAt": "2025-09-26T21:03:19Z",
          "managedBy": "system",
          "metadata": null,
          "name": "AI Tokens: usage in period",
          "paymentTerm": "in_arrears",
          "perUnitAmount": "0.01",
          "period": {
            "from": "2025-08-26T21:03:19Z",
            "to": "2025-09-26T21:03:19Z"
          },
          "quantity": "300",
          "rateCard": {
            "price": {
              "amount": "0.01",
              "paymentTerm": "in_arrears",
              "type": "flat"
            },
            "quantity": "300"
          },
          "status": "detailed",
          "totals": {
            "amount": "3",
            "chargesTotal": "0",
            "discountsTotal": "0",
            "taxesExclusiveTotal": "0",
            "taxesInclusiveTotal": "0",
            "taxesTotal": "0",
            "total": "3"
          },
          "type": "flat_fee",
          "updatedAt": "2025-08-26T14:20:30.396154-07:00"
        }
      ],
      "createdAt": "2025-08-26T21:03:19.404195Z",
      "currency": "USD",
      "featureKey": "ai_tokens",
      "id": "01K3M32GVCEZ8BTG62DX0NVBQH",
      "invoice": {
        "id": "01K3M32GV47W7RD1XR93091X2Y"
      },
      "invoiceAt": "2025-09-26T21:03:19Z",
      "managedBy": "subscription",
      "metadata": null,
      "name": "AI Tokens",
      "period": {
        "from": "2025-08-26T21:03:19Z",
        "to": "2025-09-26T21:03:19Z"
      },
      "price": {
        "amount": "0.01",
        "type": "unit"
      },
      "quantity": "300",
      "rateCard": {
        "featureKey": "ai_tokens",
        "price": {
          "amount": "0.01",
          "type": "unit"
        }
      },
      "status": "valid",
      "subscription": {
        "billingPeriod": {
          "from": "2025-08-26T21:03:19Z",
          "to": "2025-09-26T21:03:19Z"
        },
        "item": {
          "id": "01K3M32GQAE770PXQH2D2AFF3X"
        },
        "phase": {
          "id": "01K3M32GQC65X37KRT5AY3CCAN"
        },
        "subscription": {
          "id": "01K3M32GQET6ZS9ZCX5WG2K6BR"
        }
      },
      "totals": {
        "amount": "3",
        "chargesTotal": "0",
        "discountsTotal": "0",
        "taxesExclusiveTotal": "0",
        "taxesInclusiveTotal": "0",
        "taxesTotal": "0",
        "total": "3"
      },
      "type": "usage_based",
      "updatedAt": "2025-08-26T21:03:19.404195Z"
    }
  ],
  "metadata": null,
  "number": "GATHER-CU2-USD-1",
  "period": {
    "from": "2025-08-26T21:03:19Z",
    "to": "2025-09-26T21:03:19Z"
  },
  "quantitySnapshotedAt": "2025-08-26T14:20:30.354639-07:00",
  "status": "gathering",
  "statusDetails": {
    "availableActions": {
      "invoice": {}
    },
    "extendedStatus": "gathering",
    "failed": false,
    "immutable": false
  },
  "supplier": {
    "addresses": [
      {
        "country": "US"
      }
    ],
    "name": "OpenMeter"
  },
  "totals": {
    "amount": "3",
    "chargesTotal": "0",
    "discountsTotal": "0",
    "taxesExclusiveTotal": "0",
    "taxesInclusiveTotal": "0",
    "taxesTotal": "0",
    "total": "3"
  },
  "type": "standard",
  "updatedAt": "2025-08-26T21:03:19.42197Z",
  "workflow": {...}
}

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/invoice-cost

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