Click Recourse > Manage added data sources
Select the data source you would like to use to create a custom metric in and select edit
Once in edit, click +ADD A FILED in the top right corner
Name your new custom field with a name that is indicative of what you are measuring with your new field.
Add your formula into the formula field. You can type the fields you want or pick from the fields on the left hand side.
Example:
Page Comparison
Manage space
Manage content
Integrations