Hook to make tracking time till first token (TTFT) easier

[omar-scio]

Title pretty much says it. Right now, to track the TTFT, you'd have to keep local state for this being the first LLM call that the Agent makes and have some custom logic. This is likely a metric many users would want to track, so having some hook on_first_token or something in the Agent seems generally useful.

[seratch]

The platform does not plan to have this enhancement, but a possible workaround would be to enable streaming, and use on_llm_start hook + the first item start event to track the duration between them. This may not be exactly accurate, but it could be helpful at some level.