Number of characters synthesized (for character-based TTS billing).
Input text tokens (for token-based TTS billing, e.g., OpenAI TTS).
The model name (e.g., 'eleven_turbo_v2', 'sonic').
Output audio tokens (for token-based TTS billing, e.g., OpenAI TTS).
The provider name (e.g., 'elevenlabs', 'cartesia').
Duration of generated audio in milliseconds.