Improve sample timing

[asmacdo]

Currently, report interval is implemented by checking whether the absolute time, but Sample interval is implemented with a what is essentially a sleep(sample_interval).

This causes the actual sample interval to be higher by the amount of time it takes to collect the sample.

https://github.com/con/duct/blob/main/src/con_duct/__main__.py#L834

Additionally for a single sample collection we are calculating the timestamps multiple times (one for each process) which is not accurate--all of that data is collected in a single measurement and should have exactly the same timestamp.

https://github.com/con/duct/blob/main/src/con_duct/__main__.py#L454

_Originally posted by @yarikoptic in #182 (comment)