Function process_files_aggregate 

pub fn process_files_aggregate<T, F, A>(
    files: &[String],
    processor: F,
    aggregator: A,
    initial: T,
) -> Result<(T, usize)>
where
    T: Send + Clone + Sync + 'static,
    F: Fn(&str) -> Result<T> + Send + Sync,
    A: Fn(T, T) -> T + Send + Sync,

Process multiple files with simple aggregation (sum, count, etc.)

This is a simpler version for cases where you just need to aggregate simple metrics across files.