If you are only extracting metrics via cloudwatch, I don't think there are any charges for it. You will only consume DT licenses when you will be monitoring hosts by oneagents or lambda etc.
Sessions are something different, it is rum. If you want to know how many agent units (x*16gb ram) you have to go to account Settings (for saas) or to cluster management console in managed. There in license tab you will have all data needed.