Of course, you monitor the performance of your custom fields! You can choose from the custom fields you have created as another filter in the widget setup.
For example, in the Brand Embassy Support Team, we monitor how many Live chat sessions needed an email follow up.
Such sessions will have the custom field Transfers: Email
In the reporting we have set up a widget with a number of open cases, called it "transfers to email" and chose the live chat sessions with custom field email:
NOTE: You can see that even though we have other custom fields set up in our account, only the ones from the drop-down lists are visible here.
In our dashboard, we can now monitor the metric: