The disk queue currently does not implement a metrics API:

https://github.com/elastic/beats/blob/d04da481f51c3f41b814106259145042baace857/libbeat/publisher/queue/diskqueue/queue.go#L284-L286

These are metrics used by self-monitoring in the shipper, as part of this issue: #11

These metrics API was designed with the memory queue in mind, so I'm not sure to what degree these metrics make sense for the disk queue, or how hard they might be to implement. Also not sure if this is something we want done by 8.6.

CC @cmacknz