Summary

• Adds support for Vertex AI models via the LangChain callback handler.
• Fixes the issue where the callback handler was not being triggered on LLM starts (just on chat model starts).

Other comments

• I didn't add a notebook example for Vertex AI. I'll add a "native" Vertex AI example in the future, once we support tracing it directly (instead of via LangChain). The LangChain notebook example should be clear enough for now.