Add new metrics for backups: num_processed_files and processed_files_size

Why it matters

These metrics provide users with detailed insight into backup progress and performance by tracking the exact count and size of files processed. This helps in monitoring backup operations more precisely and diagnosing backup-related issues.

How to use it

After upgrading to the version that includes this feature, users can monitor the new metrics num_processed_files and processed_files_size as part of their backup monitoring tools or queries to assess backup processing details.