Interface RealtimeClientUsage

A usage/telemetry update emitted by a realtime client.

Deltas preferred: each emission SHOULD carry the incremental token counts for the response/turn that just completed (not a session-cumulative running total), matching the server-side RealtimeUsage contract — consumers accumulate. A driver whose provider only reports cumulative totals must convert to deltas before emitting.

Per-driver availability (capability, not laziness — providers without usage events simply never emit):

• OpenAI — emits per-response deltas from the GA response.done frame's usage payload.
• Gemini — emits per-turn deltas from LiveServerMessage.usageMetadata (promptTokenCount / responseTokenCount are per-response counts, not cumulative).
• ElevenLabs — the Conversational AI socket exposes no usage events; never emits.
• AssemblyAI — the streaming STT socket exposes no token-usage events; never emits.