Switching from `time` to `monotonic`

Currently we use `time` (see below), but this can be problematic. For example @pentschev and @quasiben recently tracked down an issue where system time could be off by as much as 2s (including going backwards! 👨‍🔬). It would be good to start using [`monotonic`]( https://docs.python.org/3/library/time.html#time.monotonic ) to avoid these issues.

https://github.com/dask/distributed/blob/383ea0326ae103b5d5e0b62ed9c3cb18510c5b9e/distributed/metrics.py#L80-L85