Add telemetry for Instructor.chat_completion

[vkryukov]

Here is a quick way to add telemetry to Instructor.chat_completion. It provides the following benefits:

• A standard Elixir way to do telemetry
• Captures all relevant information, including raw LLM response and time it took to respond, to help with debugging
• Can add additional measurement (e.g., number of tokens consumed) but that would require adjusting the Instructor.Adapter behavior

Let me know if you want me to modify it in any way.