[nik13]

Summary

• Add an OpenAI .NET ChatClient how-to using the upstream experimental ActivitySource and stock OTLP HTTP/protobuf exporter; no duplicate wrapper or unpublished package.
• Register the C# guide independently of the Python/JavaScript client guide and Azure OpenAI .NET guide.
• Describe the released non-streaming scope, native error-text privacy caveat, trace-only configuration, and exact project/auth headers.

Verification

• NuGet published OpenAI 2.14.0 and OpenTelemetry 1.20.1 are latest stable and pinned; native floor and latest OpenAI release are the same version.
• Offline loopback receivers: 32/32 scenarios on .NET 8 and 32/32 on .NET 10, including supported chat, errors, disabled and unsupported APIs, default/latest semconv, resource and auth headers, no duplicate model spans, and no credentials sent to OpenAI.
• Compiled the guide C# example and ran against synthetic OpenAI and OTLP endpoints on both .NET 8 and .NET 10 (2/2 each), with only endpoints overridden in the local test copy.
• Docs integration tests (24/24), TypeScript catalog check, Astro build/Pagefind passed in a network-denied sandbox. No real API key or live OpenAI call used.

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