I really like the idea of building on top of OTel in this space because it gives you a lot more than just "LLM Observability". More specifically, it's a lot easier to get observability on your entire agent (rather than just LLM calls).

I'm working on a tool to track semantic failures (e.g. hallucination, calling the wrong tools, etc.). We purposefully chose to build on top of Vercel's AI SDK because of its OTel integration. It takes literally 10 lines of code to start collecting all of the LLM-related spans and run analyses on them.