I’m trying to sum up the values of a metric and have found the ‘Sum’ attribute in the Edit Performance Measure dialogue, but it does not seem to sum up the values of my metric month on month, am I missing something here?
You seem to have misunderstood what the Aggregation parameter is for. The ‘Aggregation’ parameter is used to determine what will happen to a metric when it is viewed in a duration longer than the Calendar duration it has been set to. For example, when a metric is set as monthly and the aggregation parameter is set to Sum, when the metric is viewed using a Quarterly or Yearly calendar, the metric will be summed. Generally, percentage metrics will be set to Average, so when viewed in a longer duration calendar, you will see a sensible percentage.
The aggregation parameter does not perform a calculation month on month as you have described. If you need to do that, then you either import the aggregated number or set up a calculation to aggregate the number.