use of $ARGS in systemd unit file by prometheus package

[Andrew Spiers <andrew@andrewspiers.net>]

Prometheus takes some command line arguments to control certain
aspects of its operation.
The systemd unit file shipped with prometheus includes the line

ExecStart=/usr/bin/prometheus $ARGS

I know I can simply override the unit file with one in
/etc/systemd/system, but what if I wanted to use this variable? Where
would I set it?