docs: List LangSmith tracing integration (#197)

LangChain employee/maintainer, happy to make edits to our
docs/integrations if needed.

Author: rm-openai

docs/tracing.md

@@ -97,3 +97,4 @@ To customize this default setup, to send traces to alternative or additional bac
 -   [AgentOps](https://docs.agentops.ai/v1/integrations/agentssdk)
 -   [Scorecard](https://docs.scorecard.io/docs/documentation/features/tracing#openai-agents-sdk-integration)
 -   [Keywords AI](https://docs.keywordsai.co/integration/development-frameworks/openai-agent)
+-   [LangSmith](https://docs.smith.langchain.com/observability/how_to_guides/trace_with_openai_agents_sdk)