[doc] add mlflow tracing integration (#171)

Hi team! Congrats on the new Agents SDK launch!

This PR adds MLflow to the tracing documentation, as it supports
automatic tracing for OpenAI Agents SDK. It has also been supporting
tracing for
[Swarm](https://mlflow.org/docs/latest/tracing/integrations/swarm) as
well, so it is fantastic that the new SDK offers built-in mechanism for
3P trace providers🙂


![image](https://github.com/user-attachments/assets/d36382da-dca1-4335-a915-ca94a994e26a)

Author: rm-openai

docs/tracing.md

@@ -93,6 +93,7 @@ To customize this default setup, to send traces to alternative or additional bac
 ## External tracing processors list
 
 -   [Arize-Phoenix](https://docs.arize.com/phoenix/tracing/integrations-tracing/openai-agents-sdk)
+-   [MLflow](https://mlflow.org/docs/latest/tracing/integrations/openai-agent)
 -   [Braintrust](https://braintrust.dev/docs/guides/traces/integrations#openai-agents-sdk)
 -   [Pydantic Logfire](https://logfire.pydantic.dev/docs/integrations/llms/openai/#openai-agents)
 -   [AgentOps](https://docs.agentops.ai/v1/integrations/agentssdk)