Problem description:

• Allow users to configure lists to show a summary of selected custom field(s). Depending on custom field type different summaries could be available.

Very rough draft:

Fall all custom field types user could select to show the count of how many cards have it. For integers, the sum could also be available, and for date type range could be selected.

For integer and date, min and max could also be possible, although I personally don't see much use.