limithandler: Log queue duration in limit.concurrency_queue_ms log field

In the same vein as !5330 (merged) and !5332 (merged).

We currently have no per-RPC stats on how long an RPC spent in the concurrency limiter. This MR introduces that capability by adding a limit.queue_ms field to the logs.

It required jumping through some hoops due to middleware ordering. I can add a more comprehensive test case similar to commandstatshandler_test if desired.