fix sorted column of system table

[qjiang002]

This is a sub-component of PR #518

Fix the sorted column of System table

• The sorter selector is kept separately above the table, and the sorted column in the table is fixed to the left and the column header sorter is set to be the same as the sorting direction.
• In the sorter selector, 'Created At' is changed to 'Sorted by' for easier understanding.

[qjiang002]

Thanks @lyuyangh for the suggestion. I add a prefix box before the selector, and pin Created_At to the right because it is a default and less important sorted column compared with those metric columns.