Foreman Proxy now allows you to define STDOUT as a log output in /etc/foreman-proxy/settings.yml. On systemd this will output to /var/log/messages or whatever you have defined for the service in systemd.
Systemd manpage:
StandardOutput=
Controls where file descriptor 1 (STDOUT) of the executed processes is connected to. Takes one of inherit, null, tty, journal, syslog, kmsg, journal+console, syslog+console, kmsg+console or socket.
inherit duplicates the file descriptor of standard input for standard output.
null connects standard output to /dev/null, i.e. everything written to it will be lost.
tty connects standard output to a tty (as configured via TTYPath=, see below). If the TTY is used for output only, the executed process will not become the controlling process of the terminal, and will not fail or wait for other processes to release the terminal.
journal connects standard output with the journal which is accessible via journalctl(1). Note that everything that is written to syslog or kmsg (see below) is implicitly stored in the journal as well, the specific two options listed below are hence supersets of this one.
syslog connects standard output to the syslog(3) system syslog service, in addition to the journal. Note that the journal daemon is usually configured to forward everything it receives to syslog anyway, in which case this option is no different from journal.
kmsg connects standard output with the kernel log buffer which is accessible via dmesg(1), in addition to the journal. The journal daemon might be configured to send all logs to kmsg anyway, in which case this option is no different from journal.
journal+console, syslog+console and kmsg+console work in a similar way as the three options above but copy the output to the system console as well.
socket connects standard output to a socket acquired via socket activation. The semantics are similar to the same option of StandardInput=.
This setting defaults to the value set with DefaultStandardOutput= in systemd-system.conf(5), which defaults to journal.