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.

AI generated by Issue Triage Agent

[reachanihere]

Update: Simplified Approach

Based on feedback by @SteveSandersonMS on PR #179, we've pivoted to a simpler approach:

Instead of built-in OpenTelemetry instrumentation, the PR now adds session lifecycle hooks that enable external observability packages:

• SessionCreated event - fires when a session is created
• SessionDestroyed event - fires when a session is disposed

This keeps the core SDK lean while allowing a separate package (e.g., GitHub.Copilot.SDK.Aspire) to provide full OpenTelemetry instrumentation using:

1. These lifecycle hooks to track session creation/destruction
2. session.On(...) to subscribe to all session events (usage, tool execution, etc.)

The full OTel implementation with spans and metrics can live in that dedicated package, following the .NET Aspire pattern for observability extensions.

[MattKotsenas]

Looks like from the linked PR that OpenTelemetry has moved into the runtime, so it seems like now this issue becomes about documenting how to subscribe.

[brandonh-msft]

What I'm seeing is that when I get copilot.tool_handler events, they have no/different parents attached to them than was is active in the tool. So, if I have a Tool that makes a REST call, for example - and I've added HttpClientInstrumentation in the DI container - I do not get the REST calls parented under the tool as you'd expect. The context is completely lost and the REST call ends up under the operation span in Application Insights, for example:

I think this is because we're crossing the RPC boundary? Not sure.

[stephentoub]

This is available via the .NET SDK:

copilot-sdk/dotnet/src/Types.cs

Lines 157 to 218 in 2ac20f0

	public TelemetryConfig? Telemetry { get; set; }
	/// <summary>
	/// Creates a shallow clone of this <see cref="CopilotClientOptions"/> instance.
	/// </summary>
	/// <remarks>
	/// Mutable collection properties are copied into new collection instances so that modifications
	/// to those collections on the clone do not affect the original.
	/// Other reference-type properties (for example delegates and the logger) are not
	/// deep-cloned; the original and the clone will share those objects.
	/// </remarks>
	public virtual CopilotClientOptions Clone()
	{
	return new(this);
	}
	}
	/// <summary>
	/// OpenTelemetry configuration for the Copilot CLI server.
	/// </summary>
	public sealed class TelemetryConfig
	{
	/// <summary>
	/// OTLP exporter endpoint URL.
	/// </summary>
	/// <remarks>
	/// Maps to the <c>OTEL_EXPORTER_OTLP_ENDPOINT</c> environment variable.
	/// </remarks>
	public string? OtlpEndpoint { get; set; }
	/// <summary>
	/// File path for the file exporter.
	/// </summary>
	/// <remarks>
	/// Maps to the <c>COPILOT_OTEL_FILE_EXPORTER_PATH</c> environment variable.
	/// </remarks>
	public string? FilePath { get; set; }
	/// <summary>
	/// Exporter type (<c>"otlp-http"</c> or <c>"file"</c>).
	/// </summary>
	/// <remarks>
	/// Maps to the <c>COPILOT_OTEL_EXPORTER_TYPE</c> environment variable.
	/// </remarks>
	public string? ExporterType { get; set; }
	/// <summary>
	/// Source name for telemetry spans.
	/// </summary>
	/// <remarks>
	/// Maps to the <c>COPILOT_OTEL_SOURCE_NAME</c> environment variable.
	/// </remarks>
	public string? SourceName { get; set; }
	/// <summary>
	/// Whether to capture message content as part of telemetry.
	/// </summary>
	/// <remarks>
	/// Maps to the <c>OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT</c> environment variable.
	/// </remarks>
	public bool? CaptureContent { get; set; }
	}