Add SimpleLogger::with_monotonic_timestamps

[newpavlov]

We use simple_logger as logger for our tests. We also run MIRI for some of our tests. Unfortunately, MIRI intentionally does not emulate SystemClock with enabled isolation, so we either have to disable isolation, or use SimpleLogger::without_timestamps. The latter works, but we lose information about timings.

It would be nice if simple_logger had support for timestamps based on std::time::Instant. It could remember Instant during initialization and use Instant::elapsed for log entry timestamps.