admission: Split stats for WorkQueueMetrics

[andrewbaptist]

This commit splits the WorkQueueMetric stats into priority. Because there are a lot of new stats that might be generated as a result of this splitting, only the "important" priorities are collected and considered for each queue.

Release note: None

[cockroach-teamcity]

This change is 

[andrewbaptist]

bors r=irfansharif

[craig[bot]]

Build succeeded:

• Bazel Essential CI (Cockroach)

[andreimatei]

This patch increased the number of timeseries exported to tsdb by crdb nodes from 731 to 1739. If there's histograms in there, it might be even much worse for Prometheus. I don't think we can afford anything like it...

[andrewbaptist]

I was concerned about the impact of this change. I can easily disable all of these and will hold off on any backporting. I might need to rethink if there is a way to get the data we want without exploding stats.

[andreimatei]

I think I fooled myself when counting, sorry. I now think that this patch only took the timeseries count from 1543 to 1739. So, now quite as bad as I thought. Still significant, though.