Generate the final file path for the active file.
The default implementation yields:
final file names are
basePathname_YYYYMMDD_HHMMSS[_]where the optional
_ suffix is only present if needed
to distinguish a file from the previously finalized file.
starts at 1 and is monitonically incremented.
This hook method can be overridden.
Note, the implementation must handle the unlikely, but happens
in tests, case where files are cycling very fast (multiple per sec)
and the retention config tosses some within that same second.
I.e., avoid generating final path sequences like:
leaf_YYYYMMDD_103099
leaf_YYYYMMDD_103099_1
leaf_YYYYMMDD_103099_2
delete leaf_YYYYMMDD_103099 -- retention cnt was 2
leaf_YYYYMMDD_103099 // should be _3