How to report on the last person who updated an invoice prior to it being validated?

Summary:

Have a need to report on AP clerk invoice count metrics and would like to know of a field other than Last Updated By to determine who was responsible for processing an invoice.

Content (please ensure you mask any confidential information):

We see there is a Last Updated By field for an invoice but as the invoice continues to through its life cycle, events such as payments also update this field. We would like to be able to report on how many unique invoices our AP Clerks touch and the only cutoff we can think of is that after the invoice is validated, the invoice typically leaves the AP Clerk's hands. Is there any field or combination of fields that can be used when reporting on invoice counts that will help us track such a metric?