Add session lifecycle hooks for observability

[reachanihere]

Summary

Add session lifecycle events to enable external observability integration.

Based on feedback, OpenTelemetry instrumentation is better suited for a separate Aspire package rather than being built into the core SDK. This PR adds the minimal hooks needed to enable that external integration.

Changes

New Events on CopilotClient

Event	Description
SessionCreated	Fires when a session is created, provides CopilotSession instance
SessionDestroyed	Fires when a session is disposed, provides session ID

Usage

var client = new CopilotClient();

client.SessionCreated += session =>
{
    // Subscribe to session events for telemetry
    session.On(evt =>
    {
        // Handle AssistantUsageEvent, ToolExecutionStartEvent, etc.
    });
};

client.SessionDestroyed += sessionId =>
{
    // Cleanup telemetry resources
};

This enables external packages (e.g., GitHub.Copilot.SDK.Aspire) to hook into session lifecycle and use session.On(...) to capture all events for OpenTelemetry instrumentation.

Closes #181

[Copilot]

Pull request overview

This PR adds opt-in OpenTelemetry instrumentation to the .NET SDK for distributed tracing and metrics, following OpenTelemetry GenAI Semantic Conventions.

Changes:

• Adds telemetry infrastructure with ActivitySource and Meter for spans and metrics
• Implements SessionTelemetryTracker to convert session events into OpenTelemetry spans
• Integrates telemetry tracking into the CopilotSession lifecycle
• Adds comprehensive unit tests (10 tests) for telemetry functionality
• Documents the observability features in README with examples

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
dotnet/src/Telemetry/CopilotTelemetry.cs	Main telemetry class providing ActivitySource, Meter, and metric recording methods with opt-in enable/disable logic
dotnet/src/Telemetry/OpenTelemetryConstants.cs	Defines constants for span names, attribute names, and metric names following GenAI semantic conventions
dotnet/src/Telemetry/SessionTelemetryTracker.cs	Converts session events to OpenTelemetry spans, managing lifecycle of session, turn, tool, subagent, hook, and inference activities
dotnet/src/Session.cs	Integrates telemetry tracker into session, instantiating it on construction and disposing on cleanup
dotnet/src/GitHub.Copilot.SDK.csproj	Adds OpenTelemetry.Api package reference and InternalsVisibleTo for test project
dotnet/test/TelemetryTests.cs	Comprehensive unit tests covering all major span types and telemetry scenarios
dotnet/README.md	Documents how to enable and configure OpenTelemetry, lists available spans and metrics, includes example trace

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

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[SteveSandersonMS]

Thanks for contributing this, @reachanihere!

Rather than integrating otel concepts/dependencies into the SDK directly here (and diverging from the other-language SDKs), the way we'd normally approach this is by suggesting there be an Aspire-specific package for the GitHub Copilot SDK (e.g., Aspire.GitHub.Copilot) and that's where the otel concepts would be added.

Of course for this to be possible, we need CopilotClient here to have the necessary extensibility so that other external code can plug in callbacks to know what telemetry to emit. My guess is that's almost covered already - if you have access to a session instance, you can already call .On(...) to get notifications of all its events.

Perhaps the one missing piece is some way to register a callback on CopilotClient to get notifications when sessions are created and destroyed, so you can also hook into their events globally. If we added that, do you think that would unlock what's needed for this scenario?

[reachanihere]

Thank you a lot for your feedback, @SteveSandersonMS! Totally agree, keeping the SDK lean and letting observability live in a separate Aspire package would be a better approach.

I think the session lifecycle hooks would unblock this scenario. I also verified that all the events I was using (turns, tools, inference, usage, etc.) are already exposed via session.On(...), so the only missing piece would be knowing when sessions are created/destroyed.

Would it be ok if I update this PR to just add those hooks (and removing all OTel implementation)? And for the Aspire package itself, is that something your team would maintain alongside the SDK?

[SteveSandersonMS]

@reachanihere Yes, that sounds great. Thanks!

And for the Aspire package itself, is that something your team would maintain alongside the SDK?

Not currently. We're trying to keep our focus more tightly defined.

[reachanihere]

@reachanihere Yes, that sounds great. Thanks!

And for the Aspire package itself, is that something your team would maintain alongside the SDK?

Not currently. We're trying to keep our focus more tightly defined.

Thanks @SteveSandersonMS, request to please review the updated PR

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

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