Improve OpenAI tracing and implement metrics

[lmolkova]

Partner [Ge] ask: improve observability for OpenAI SDK

• OpenAI SDK should enrich traces with additional attributes, suggested semconv: open-telemetry/semantic-conventions@main...lmolkova:semantic-conventions:openai
• And emit metrics

Prototype: Azure/azure-sdk-for-net@main...lmolkova:azure-sdk-for-net:openai-hackathon

[lmolkova]

See also https://github.com/cartermp/opentelemetry-instrument-openai-py

[lmolkova]

See also https://github.com/openai/openai-dotnet/blob/main/docs/observability.md and https://github.com/open-telemetry/semantic-conventions/blob/main/docs/gen-ai/gen-ai-spans.md