Add OpenTelemetry support for .NET SDK

[reachanihere]

Add distributed tracing and metrics instrumentation to the .NET SDK using OpenTelemetry.

Features:

• Opt-in via AppContext.SetSwitch("GitHub.Copilot.EnableOpenTelemetry", true) or environment variable
• Spans: copilot.session, copilot.turn, copilot.tool_execution, copilot.subagent, copilot.hook, copilot.inference
• Metrics: token usage, cost, tool executions, errors, duration
• Follows GenAI Semantic Conventions

Benefits:

• End-to-end visibility into Copilot SDK operations
• Integration with any OpenTelemetry-compatible backend (Jaeger, Zipkin, Azure Monitor, Datadog, etc.)
• Consistent with OpenAI SDK's opt-in telemetry pattern

[reachanihere]

Created #179

[github-actions]

Thank you for opening this issue! 🙏

I've labeled this as:

• sdk/dotnet — Specific to the .NET SDK
• enhancement — Feature request for OpenTelemetry support

This is a well-structured proposal with clear features and benefits. The opt-in approach and alignment with GenAI Semantic Conventions look great. The maintainers will review this enhancement request.

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