Add OpenTelemetry GenAI instrumentation to Copilot SDK

Introduced built-in OpenTelemetry tracing and metrics for agent turns and tool executions, following GenAI semantic conventions (v1.40).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

README.md

@@ -109,6 +109,7 @@ Please use the [GitHub Issues](https://github.com/github/copilot-sdk/issues) pag
 
 - **[Getting Started](./docs/getting-started.md)** – Tutorial to get up and running
 - **[Authentication](./docs/auth/index.md)** – GitHub OAuth, BYOK, and more
+- **[OpenTelemetry Instrumentation](./docs/opentelemetry-instrumentation.md)** – Built-in tracing and metrics following GenAI semantic conventions
 - **[Cookbook](https://github.com/github/awesome-copilot/blob/main/cookbook/copilot-sdk)** – Practical recipes for common tasks across all languages
 - **[More Resources](https://github.com/github/awesome-copilot/blob/main/collections/copilot-sdk.md)** – Additional examples, tutorials, and community resources