v.25.5Improvement

Add metrics on failing merges

Add metrics on failing merges. #79228 (Miсhael Stetsyuk).
Introduces new metrics to monitor failed merge operations in ClickHouse.

Why it matters

This feature helps users identify and track merge failures in their ClickHouse tables, improving observability and enabling quicker troubleshooting of data consistency and performance issues caused by merges that do not complete successfully.

How to use it

Users do not need to perform any manual configuration; the new metrics on failing merges are automatically collected and exposed via ClickHouse's metrics interface, where they can be monitored using standard observability tools.