Hi Jonathan,
We have two ways , one is to check whether the target mbean totalConnection is an exposed jmx mbean already on server, and i would request you to check in node dashboard -> JMX as stated in doc link https://docs.appdynamics.com/display/PRO41/Configure+JMX+Metrics+from+MBeans
- If the target mbean not exposed than as of now i do not see any direction that will give sum of two different mbeans but we can use rest api to gather metrics from metric browser and report back to metric browser using machine agent custom metrics feature so that it could be used in dashboard
https://docs.appdynamics.com/display/PRO41/Use+the+AppDynamics+REST+API
https://docs.appdynamics.com/display/PRO41/Extensions+and+Custom+Metrics
- Though we do not have an option to report sum of two different metrics from metric browser but if target is to get alert when the sum crosses a value, than you can use existing "create custom expression" feature in Alert & Respond -> Health rule -> add health rule -> in edit view under condition section select "custom expression" where we can get metric1+metric2 as expression
Let us know if that information helps