local disk metrics

[iximeow]

in looking at #1073 I noticed two things:

• while all the plumbing is there for local disks to have metrics, we only don't because we gate adding the consumer on the block device being Crucible
• the metrics consumer is one Arc contended upon by all disk threads

While we could pretty easily turn on local disk metrics on a whim (we should just move the block_dev.set_metric_consumer line), doing so would add the same kind of problem as #1003 solved: one Arc whose refcount is contended over by basically every (non-vCPU) thread in Propolis. so we really should not do that without fixing the issue and measuring relative impact.

[hawkw]

Yeah the big Arc thing is really not my favorite and has in fact bothered me for Kinda A While.

[iximeow]

dupe of #949, ope