Composite Metrics are a useful way to aggregate existing metric definitions.
In order to build a Composite Metric, you will need to build a Custom Definition metric.
Here you can search for the metrics you would like to aggregate, e.g Impressions
Once you have selected your desired metrics hit 'OK'. This metric definition will now add the total impressions across the desired platforms.
You can also use composite metrics to calculate CPA, CPC etc.
