Add OTel GenAI traces:
- `create_agent`
- `invoke_agnet`
- `execute_tool`
Introduces context manager helpers to create these traces. The helpers
also serve as instrumentation points for other instrumentation
libraries.
Resolves#6644
## Why are these changes needed?
This PR adds an example which demonstrates how to build a streaming chat
API with multi-turn conversation history and a simple web UI for handoff
multi-agent design pattern.
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Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>