[TODO] Track number of files ready to be archived in pg_stat_archiver

Hello,

Attached patch implements the following TODO item :

Track number of WAL files ready to be archived in pg_stat_archiver

However, it will track the total number of any file ready to be
archived, not only WAL files.

Please let me know what you think about it.

Regards.
--
Julien Rouhaud
http://dalibo.com - http://dalibo.org