File based backends should have the ability to send IO metrics to Oximeter

[leftwo]

File backeds, just like Crucible backends, should have the ability to send IO metrics to Oximeter.

[iximeow]

note for future-us, I think the straightforward approach of "just add a metrics consumer and go" will be problematic for file-based backend performance in practice: #1095 (comment)

we probably want metrics collected per worker thread or per SQ/CQ and aggregated more infrequently so we're not contending a few words across the whole processor in the large VM/heavy workload cases.