Metrics collection for TicketHolder subclasses are currently structured to make it easier to collect metrics for the relatively complex case of the PriorityTicketHolder, which wants to report metrics for three virtual queues it maintains. This makes it harder to introduce metrics for a new subclass. This ticket covers reimagining metrics collection for TicketHolders (and the related AdmissionContext), optionally reducing the number of atomics used in the process.