Add observability for OpenAI conversations

When https://github.com/coder/aibridge/issues/83 is done, it may be useful to have some visibility into conversations as they can be created with some [initial items](https://platform.openai.com/docs/api-reference/conversations/create#conversations_create-items) or [items can be added](https://platform.openai.com/docs/api-reference/conversations/create-items) to existing conversations.

Exact solution / feature set to be decided.