Add instrumentation for simple convenience chat calls

[lmolkova]

This PR builds foundation for OpenAI SDK tracing and metrics instrumentation (using Otel-compatible .NET primitives).

It's limited to convenience ChatClient methods without streaming. The PR implements instrumentation according to OpenTelemetry GenAI semantic conventions.

The intention is to add instrumentation to other methods and client types and evolve it along with OTel GenAI semantic conventions.

TODO (in this PR):

• [ ] add samples/docs
• [ ] add experimental feature-flag required to enable instrumentation - we don't know when OTel semantic conventions will be stable and expect breaking changes.

TODO (in next PRs):

• [ ] add instrumentation to streaming calls and protocol methods
• [ ] track prompts and completions in events
• ...