Changes:
- Include coprocessor suspend time in the existing copr wait time metric
- Add panels for semaphore waiting time and the number of copr tasks
  waiting on the semaphore.

Background: 
- `total_suspend_time` measures how long a coprocessor task is not
actually being processed. This can be prolonged for several reasons,
including YATP scheduling wait and waiting for the concurrency limiter
semaphore. Previously, `total_suspend_time` was only visible in TiKV’s
slow query logs.
- Semaphore waiting is one of the main contributors to long suspend
times. The concurrency limiter exists to prioritize completing a limited
number of heavy tasks rather than spreading work thin across too many
tasks. In this mechanism, when a copr task runs for more than 5ms, it
must acquire a semaphore permit before continuing. The total number of 
permits defaults to the number of CPU cores.

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