Hello Francis Friel,
Welcome to the Microsoft Q&A and thank you for posting your questions here.
There is no separate or dedicated Unified Monitoring Agent specifically for Azure Arc that has been officially announced on the roadmap for now. However, the Azure Monitoring Agent (AMA) has been integrated with Azure Arc, which allows monitoring of hybrid and multi-cloud environments.
https://techcommunity.microsoft.com/t5/azure-governance-and-management/announcing-azure-monitoring-agent-support-in-azure-landing-zones/ba-p/4172438 So, Microsoft is advancing unified monitoring for hybrid cloud environments through the integration of the Azure Monitoring Agent (AMA) with Azure Arc.
I hope this is helpful! Do not hesitate to let me know if you have any other questions.
Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful.