When I add an interface/object to a map on Orion Atlas, the metrics that it monitors is already defined - is there a way to add custom metrics to monitor rather than the pre-defined ones?
Not directly, you can call in variables for any column on the interfaces table in the database. One trick that you COULD do is to create a custom property, and write an alert rule that would update that custom property with the data for your custom metric. Or if you are comfortable in SQL directly you could write a trigger or similar to update the interface custom property whenever the metric is polled (I'm assuming this would be from something like a UNDP)