We don't have visibility on the disk fragmentation for a given file. On way to get there is to use the following formula defined in WT-16684:

FI = (1 - (1 / N)) * (Total Free Space / Total File Size) * 100
(Where 'N' is the total number of free extents in the file).

We want to get N through stats for the alloc and avail extent lists.

The work is to create the relevant stats so we can observe N in the FTDC and update logs so we have the number of extent lists and the fragmentation ratio.