.NET: expose OpenTelemetryAgent.DefaultSourceName

[Yashvant Mahadev Hange (YashvantHange)]

Motivation & Context

ADR 0003 documents how to subscribe to agent telemetry:

using var tracerProvider = Sdk.CreateTracerProviderBuilder()
    .AddSource(AgentOpenTelemetryConsts.DefaultSourceName)
    .AddConsoleExporter()
    .Build();

That snippet does not compile against the shipped package. AgentOpenTelemetryConsts was renamed to OpenTelemetryConsts in #356, and the type is internal, so DefaultSourceName is unreachable from outside the assembly. Consumers who want to receive invoke_agent spans under the default source have to hardcode the literal "Experimental.Microsoft.Agents.AI", which fails silently if that name ever changes. The repository's own tests hardcode the same literal.

Description & Review Guide

• What are the major changes?

  OpenTelemetryAgent gains a public const string DefaultSourceName that forwards to the existing internal OpenTelemetryConsts.DefaultSourceName. The constant is placed on the type that emits the spans, so OpenTelemetryConsts and the gen_ai.* attribute keys remain internal, preserving the Microsoft.Extensions.AI symmetry that ADR 0003 lists as a design goal. The ADR usage snippet is corrected to reference the constant that now exists. A unit test pins the constant to the literal source name so a rename cannot silently break subscribers.

• What is the impact of these changes?

  Purely additive. Release builds of Microsoft.Agents.AI succeed across all five target frameworks with no warnings, and Package Validation reports no compatibility diagnostics against the published baseline. No existing behavior changes.

• What do you want reviewers to focus on?

  Whether OpenTelemetryAgent is the placement you want for the constant. Moving it is a one-line change if you would prefer it elsewhere. The narrower question of whether to also make the gen_ai.* attribute keys public is deliberately left out of this change, since it is a larger API commitment.

Related Issue

Fixes #6452

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Exposes the default OpenTelemetry activity source name for external subscribers and updates guidance and tests accordingly.

Changes:

• Adds OpenTelemetryAgent.DefaultSourceName.
• Updates the ADR usage example.
• Tests the exposed source name.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
dotnet/src/Microsoft.Agents.AI/OpenTelemetryAgent.cs	Exposes the default telemetry source name.
dotnet/tests/Microsoft.Agents.AI.UnitTests/OpenTelemetryAgentTests.cs	Verifies the exposed value.
docs/decisions/0003-agent-opentelemetry-instrumentation.md	Uses the public API in the example.

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[Yashvant Mahadev Hange (YashvantHange)]

@microsoft-github-policy-service agree