[opentelemetry-instrumentation-genai-smolagents] instrument agent runs - #403

Description

Adds invoke_agent instrumentation and the duration metric for smolagents MultiStepAgent.run, streaming and non-streaming. The span records the agent name and description, the model, tool and managed-agent definitions, the finish reason, and the task and final answer when content capture is enabled.

Known gaps:

• Agent steps do not get separate spans. See Adding ReAct Iterations Spans in Reasoning-Acting Agents semantic-conventions-genai#81.
• CodeAgent with the default executor_type="local" writes code and runs it in a worker thread. OpenTelemetry context isn't copied into the thread, so a managed agent called from that code cannot see the parent span.

Part of #141

Type of change

• New feature (non-breaking change which adds functionality)

How has this been tested?

uv run tox -e py310-test-instrumentation-genai-smolagents-oldest
uv run tox -e py314-test-instrumentation-genai-smolagents-latest
uv run tox -e py314-test-instrumentation-genai-smolagents-conformance
uv run tox -e precommit
uv run tox -e typecheck
uv run tox -e readme

Checklist

See CONTRIBUTING.md
for the style guide, changelog guidance, and more.

• Followed the style guidelines of this project
• Changelog updated if the change requires an entry
• Unit tests added
• Documentation updated