Add metrics for queued jobs in IO thread pool

Why it matters

This feature provides visibility into the workload and performance of the IO thread pool by tracking how many jobs are waiting in the queue. It helps users understand IO thread utilization and identify potential bottlenecks.

How to use it

The metrics are automatically collected and exposed. Users can monitor these metrics through the existing ClickHouse metrics interface or integrate them with their monitoring systems to observe the IO thread pool queue length.