Add trace counters for the number of pending tasks in network scheduler

This CL adds `TRACE_COUNTER` to track the number of pending tasks in the
Default and High priority queues of the network task scheduler.

This allows for visualizing the queue lengths in Perfetto, which will
help in diagnosing potential scheduling issues.

Bug: 421051258
Change-Id: Ie92688b633a333b7ea1f977433c5cc45e845d96f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6764148
Commit-Queue: Hayato Ito <hayato@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1488598}