`recon:node_stats` - Issue in number of reductions

[rewritten]

The function node_stats(N, Interval, FoldFun, Init) has an issue with the returned number of reductions. This is because erlang:statistics(reductions) returns a tuple with {Total, Count_since_last_call} but those calls are global.

recon relies on calling this function twice and using the count, but if any other process in the same VM calls erlang:statistics(reductions), then the returned number is much lower than the real number.

You can see it for instance calling recon:node_stats_list(1, 1000) in a task and, just before one second has passed, call erlang:statistics(reductions) manually. Alternatively, call recon:node_stats_list(1, 1000) twice, concurrently: one of the two stats will have a very very low number of reductions.

Probably reductions should instead be computed as delta between the Total (first element).

[ferd]

yeah that's a good idea for a more accurate result. I never had to run into that issue in the past (recon used to be a bunch of snippets coworkers and I just ran in the shell), but with the amount of tooling that can run in the background, the risk of higher interference has grown over time.