How do I capture the value of tokens used ($usage.total_tokens) when using the new AI Agents? I was able to do this previously using the retrieveRun call, but can't see how to do this with the new Agents feature. Since I have to pay for token usage when my users interact with AI on my website, it is extremely important to track how many tokens my users have used so I can charge them accordingly. My website model is a pay-for-usage structure rather than a subscription structure