Add tasks/memory metrics for distributed/buffer schedule pools

Why it matters

This feature provides enhanced observability by exposing detailed metrics about task execution and memory consumption in distributed and buffer schedule pools. It helps users monitor and diagnose scheduling performance and resource usage more effectively.

How to use it

After upgrading to the version including this feature, users can access the new metrics via the existing metrics interfaces, such as system.metrics and monitoring tools integrated with ClickHouse. No additional configuration is required to start collecting these metrics.