admission: improve descriptions of metrics in the admission pkg

We a few fair share of metrics with vague descriptions. We should add a bit more to allow the consumer of those metrics understand what exactly the metric represents.

Even though a majority of these metrics are meant to be internal, better descriptions will help other engineers and PMs know what it is.

Example:
`admission.requested.kv-stores.high-pri` -> "Number of requests". This could likely say something along the lines of "Total number of requests for admission to the store work queue".

cc @dshjoshi

Jira issue: CRDB-45539