admission: additional obs for items that wait in the wait queue

Currently, when an item waits in the work queue, we track the wait duration and expose it as a histogram tracked by work queue type. We also add a structured log to the tracing span (if one exists) for each individual work item that waited in queue.

An [internal discussion](https://cockroachlabs.slack.com/archives/C01SRKWGHG8/p1730121538919249) led to a suggestion that we should additionally return this information to the original source of the request that waited in an AC queue to help the caller aggregate metrics about its work items waiting in AC.

Jira issue: CRDB-43686