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i'm measuring the ttft latency for gemini's realtime llm, and sometimes see a value of -1, what could this mean?

if ev.metrics.type == "eou_metrics":
            <http://logger.info|logger.info>("Aggregating EOU metric")
            agg["eou"].append(ev.metrics.end_of_utterance_delay)
        elif ev.metrics.type == "llm_metrics":
            <http://logger.info|logger.info>("Aggregating LLM TTFT metric")
            agg["ttft"].append(ev.metrics.ttft)
        elif ev.metrics.type == "tts_metrics":
            <http://logger.info|logger.info>("Aggregating TTS TTFB metric")
            agg["ttfb"].append(ev.metrics.ttfb)
        elif ev.metrics.type == "stt_metrics":
            <http://logger.info|logger.info>("Aggregating STT duration metric")
            agg["stt"].append(ev.metrics.audio_duration)
        elif ev.metrics.type == "realtime_model_metrics":
            <http://logger.info|logger.info>("Aggregating Realtime LLM TTFT metric")
            agg["ttft"].append(ev.metrics.ttft)
        metrics.log_metrics(ev.metrics)

-1 indicates that no audio token was sent: <https://github.com/livekit/agents/blob/main/livekit-agents/livekit/agents/metrics/base.py#L117>

So, for whatever reason, the LLM did not produce any tokens