docs: add arize-phoenix to tracing documentation (#169)

Author: rm-openai

docs/tracing.md

@@ -92,6 +92,7 @@ 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)
 -   [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)