Think: Improve Otel hooks so we can wrap the whole turn in `startActiveSpan`

It's me again, sorry. That's a proposal, not a bug.

So, I'm trying to get visibility on my agents using an Otel based tool like https://latitude.so. The challenge is that in Otel, you want to use `startActiveSpan` so you create a hierarchy (https://www.honeycomb.io/blog/startspan-versus-startactivespan-opentelemetry) of spans. The challenge is that there is nothing to wrap since the only hooks that Think provide are lifecycle hooks (before/after). I can use `startSpan` using those lifecycle hooks, but then the instrumented spans from AI sdk coming from the inner `streamText` won't belong to the parent span (which I want to give context about the user and whatnot).

I hope that makes sense to you. Basically, if instead of hooks Think would expose a middleware-like API instead, we could wrap things (like telemetry) around it.