curl --request GET \
  --url https://api.example.com/api/billing/summary/{agent_id}

{
  "total_calls": 123,
  "billing": {
    "stt": {
      "deepgram": 0,
      "sarvam": 0
    },
    "tts": {
      "elevenlabs": 0,
      "sarvam": 0
    },
    "llm": {
      "openai": 0,
      "openrouter": 0
    }
  },
  "grand_total": 123,
  "currency": "USD"
}