[CELEBORN-1812] Distinguish sorting-file from sort-tasks waiting to b…

…e submitted

### What changes were proposed in this pull request?

Current implementation uses `
shuffleSortTaskDeque.size()` as current sorting file count.This value might be more appropriately described as the sort tasks waiting to be submitted to `fileSorterExecutors`. And the actual current sorting file number ( doing some disk-io operation etc) should be get from `sortingShuffleFiles`.

### Why are the changes needed?

Add metrics to monitor current sorting files which is making disk-io operations.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

![image](https://github.com/user-attachments/assets/6ffed37e-ad12-4d8d-a4aa-2b2695a92168)

Closes #3040 from Z1Wu/fix/sorting_file_metrics.

Authored-by: wuziyi <[email protected]>
Signed-off-by: Shuang <[email protected]>