Creating backup history files for backups taken from standbys

Hi all,

In the recent thread related to a bug in pg_stop_backup when waiting
for WAL segments to be archived, it has been mentioned that it would
be nice to get backup history files generated as well on standbys:
https://www.postgresql.org/message-id/CA+Tgmobh3=quCzugFP5yA3-z5uzz=YKZ7arnoApJoPN7=0H8TQ@mail.gmail.com

Backup history files are not essential to backups, but they are for
debugging purposes, and it has bugged me hard that we should have a
consistent behavior in this area, particularly because with
archive_mode = always they can be archived by a standby.

The previous thread has ended with 52f8a59d, which addressed the
original problem but discarded the backup history file generation
during recovery. Attached is a patch to fix this inconsistency, parked
in next CF. I am not arguing for this change as a bug fix, but as an
improvement for PG11.

Thoughts?
--
Michael