Add metrics to track time from worker spawn start until each proof
worker marks itself as available (after initialization, before
processing first job).
Changes:
- Add storage_worker_spawn_to_available_duration histogram
- Add account_worker_spawn_to_available_duration histogram
- Capture timestamp before spawning workers
- Pass timestamp to worker constructors
- Record elapsed time when workers mark themselves available
These metrics help identify worker initialization bottlenecks and
understand parallel proof computation startup performance.
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