Hot backup causes uncontrolled growth of WiredTigerPreplog files

WiredTiger engine is not using prepared log files for log file allocation when hot backup is in progress. The relevant logic is implemented in function __log_newfile.
Due to this logic prep_missed counter is increased which causes log file server thread (__log_file_server) to bump counter log_prealloc. log_prealloc controls number of pre-created log files (WiredTigerPreplog.*).

So with every new hot backup log_prealloc counter grows more and more thus creating more and more WiredTigerPreplog.* files in journal directory. For example, after a month of daily hot backups we have about 370 prep log files (each of 100MB => 37GB of junk) which I even cannot delete premanently as the log file thread re-creates them to the count of log_prealloc property.

My proposed solution is not to increment prep_missed counter when hot backup is active since we deliberately not using those prep files.