Add `id` and `finish_reason` to `ModelResponse`

[alexmojaki]

These fields would be used to populate gen_ai.response.id and gen_ai.response.finish_reasons in https://opentelemetry.io/docs/specs/semconv/gen-ai/gen-ai-spans/#genai-attributes

[izzyacademy]

@alexmojaki please take a few minutes to add a description to your issue.

[nakranivaibhav]

I would like to take this up. Some context would be really helpful.