Report Hosts Running Distributed DDL Queries and Activate Replicas in DDLWorker

Why it matters

This feature addresses the need to track and report which hosts are currently executing distributed DDL queries in a ClickHouse cluster. By creating a replica_dir and marking replicas as active in DDLWorker, it improves visibility and coordination of distributed schema changes, helping to monitor and potentially troubleshoot DDL operations across replicas.

How to use it

This feature is integrated into the DDLWorker mechanism. Users benefit from improved reporting of active replicas during distributed DDL queries without additional manual configuration. The system automatically creates the replica_dir and tracks active replicas when distributed DDL queries are run.