Please add support for MONOTONIC_USEC= to sd_reloading()

MONOTONIC_USEC= is used together with RELOADING=1 for Type=notify-reload services, which were added a couple of years ago.

Please consider including MONOTONIC_USEC=<system monotonic clock in microseconds> in the same string as RELOADING=1 in the sd_reloading() helper. It can always be sent unconditionally, even on very old systems it will just be ignored if not supported.

https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html#MONOTONIC_USEC=%E2%80%A6

Thanks!