Fine performance metrics: Break down idle time on the Worker

[crusaderky]

• Part of Fine performance metrics meta-issue #7665

As of #7586, the sum of Worker.digests_total[("execute", *, *, "seconds")] is equal to the time spent executing tasks, multiplied by the number of threads on the worker.

There's a big chunk of extra time that is not counted, which is:

• Time the worker spent with idle threads because it was paused.

Worker.digests_total["execute", "n/a", "paused", "seconds"] = min(
   number of threads - number of tasks in executing/cancelled/resumed state,
   number of tasks in ready or constrained state,
) * T

where T is the time between changes in any of the variables in the formula.
Note that we would not record the task prefix here, unlike in other execute digests.
Note that a worker may be paused and not accrue any paused time, because there are tasks still running.

• Time the worker spent with idle threads because of resource limits

Worker.digests_total["execute", "n/a", "constrained", "seconds"] = max(0, min(
   number of threads - number of tasks in executing/cancelled/resumed state,
   number of tasks in constrained state
) * T - paused time)

• Time the worker spent with idle threads because it was fetching data. This is different from the sum of Worker.digests_total[("gather-dep", *, "seconds")] as it should exclude the time where dependency gathering and execution where properly pipelined. In other words, this time should be defined as

Worker.digests_total["execute", "n/a", "gather-dep", "seconds"] = max(0, min(
   number of threads - number of tasks in executing/cancelled/resumed state,
   number of tasks in waiting state,
) * T
- paused time 
- constrained time
)

• time the worker spent with idle threads because it was waiting for more content from the scheduler.
  This should be defined as

Worker.digests_total["execute", "n/a", "idle", "seconds"]  = max(0, 
    (number of threads - number of tasks in executing/cancelled/resumed state) * T
    - paused time
    - constrained time
    - gather time
))

With the above additions, the sum of Worker.digests_total[("execute", *, *, "seconds")] should accumulate to (number of threads * worker uptime) by construction when there are no tasks currently running.

The above formulas are a very quick draft and should be reviewed for correctness.

[crusaderky]

#7938 introduces a metric on the spans that measures the total of the idle time. This is superior to just beaming the same information from the workers as it is aware of tasks running everywhere on the scheduler. This issue should be reviewed/redesigned in light of this.