Parallel replicas: Introduce 'ParallelReplicasUsedCount' profile event

Why it matters

This feature enables users to monitor how often the parallel replicas mechanism is utilized during query execution, providing insights into query distribution and load balancing across replicas. It helps improve understanding and tuning of replication parallelism for better performance.

How to use it

Users can access the new profile event <code>ParallelReplicasUsedCount</code> through the system profile events to observe the count of times parallel replicas are used. This feature is experimental and may require enabling related experimental settings in the ClickHouse configuration.