If you're trying to add a custom metric to a clips and its not showing next to the clip, or in the metrics summary, its likely that metric has already been hidden from the book.
If you have chosen to hide the metrics summary page altogether, click to unhide it.
In this example, Circulation has been hidden from showing in the book. You can check for any hidden metrics by clicking on the "+ hidden metrics" bar on the metrics summary page
Check the tick box for the metrics you would like to show in the book and click on update. They'll now be visible (you can always choose to hide the summary page again after).
If you need more information our FAQ on how to show and hide metrics should help.