public int64 get_monotonic_time ()
Queries the system monotonic time.
The monotonic clock will always increase and doesn't suffer discontinuities when the user (or NTP) changes the system time. It may or may not continue to tick during times where the machine is suspended.
We try to use the clock that corresponds as closely as possible to the passage of time as measured by system calls such as poll but it may not always be possible to do this.
the monotonic time, in microseconds