feat(taskworker): Add child fetch wait duration metric

[enochtangg]

Description

Adds taskworker.worker.child.fetch_wait_duration distribution metric of how long each child blocks in child_tasks.get() waiting for work.

Question we'd like to answer:
When a child is idle, is it because the parent has nothing to give it, or because it can't get at work that's already sitting in the queue?

In process-segments-push (s4s2), children are idle ~55% of the time while child_tasks.size averages 8.97. A get() on a queue that deep should return in quickly. We can't currently tell which of those is wrong.

Existing metrics don't answer it. child_task_queue_empty only fires after a full 1s timeout, so it counts famines and misses the normal case. Occupancy gives the aggregate but not the shape, and the shape is what identifies the cause:

Signature	Cause
Long waits while child_tasks.size > 0	_rlock contention, children serializing on one queue lock
Long waits only while size == 0	Supply-limited upstream of the parent

That distinction decides how to update the autoscaling metric