.NET: Investigation: .NET OTel auto-instrumentation convention for Agent Framework

@rogerbarreto is already working on this.

Since May 14, 2026.

## Objective

Big OTel rock in .NET land. Tribe pattern say: `AddXxxInstrumentation()` on provider, kill switch env var, separate package. Tribe use ASP.NET Core, Azure SDK, EF Core. Same shape.

Agent Framework today: must wrap each agent by hand with `UseOpenTelemetry()`. No tribe pattern. No kill switch. No one-line activate.

Need investigate. Not code yet. Lay stone first:

- Where instrumentation live? Core? New `Microsoft.Agents.AI.OpenTelemetry` package? Contrib `OpenTelemetry.Instrumentation.AgentFramework` (open-telemetry tribe own `OpenTelemetry.*` prefix on nuget, can't take)?
- How activate? DI auto-decorate `AIAgent`? Provider builder extension? Both? Idempotent?
- Kill switch env var name? `OTEL_DOTNET_AGENTFRAMEWORK_INSTRUMENTATION_ENABLED`?
- Source name rename? Today `Experimental.Microsoft.Agents.AI`. Tribe say `Microsoft.Agents.AI`.
- Workflows source `Microsoft.Agents.AI.Workflows` same package?
- Long term: move emit into base `AIAgent` BCL `ActivitySource` listener-driven (like `HttpClient`)? Skip wrapper?
- Future: contrib package shim post-GA, post-stable gen-AI semconv?

Land ADR with options + decision before any code. Existing ADR 0003 stay. New ADR add on top.

## Outcome

ADR `docs/decisions/0026-agent-otel-auto-instrumentation.md` proposed, reviewed, accepted. Then implementation issue spin off.

## Refs

- ADR 0003 (current `OpenTelemetryAgent` wrapper)
- https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/versioning-and-stability.md
- https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/blob/main/docs/config.md
- https://www.nuget.org/packages/OpenTelemetry.Instrumentation.AspNetCore