Add proportional resident memory size

[PierreF]

This PR fix #140 by adding two additional memory_bytes memtype:

• proportionalResident
• proportionalSwapped

I've added a command flag to enable/disable smaps information gathering as this cause notable CPU impact (when gathering all 480 processes - and 3000 threads - on my laptop: without process-exporter consume ~9% of CPU, with it consume ~30% of CPU). It's disabled by default.

Edit: updated to use prometheus/procfs/pull/286. This has much lower CPU impact.
Edit: It still include #141 so it may be merged after #141 (or supersede it).

This is WIP because:
* It include #141 which should be merged before (or #141 declined and only this one merged)
* It rely on prometheus/procfs#280 and prometheus/procfs#281. Before those PRs are merged, it's using my own fork of procfs.

I'll update this PR when other PRs get merged.

[PierreF]

Updated to drop the replace in go.mod.
Like for #141 I switch to the commit which merged prometheus/procfs#286

I've kept the toggle flag, but the CPU cost if now much lower since procfs use smaps_rollup. I've run three process-exporter with and without the -gather-smaps flag and the last is before using smaps_rollup (e.g. using prometheus/procfs#281) :

• Without gathering smaps, process-exported used 25.7 seconds of CPU (for 25 collections)
• With gathering smaps (using smaps_rollup), it used 34.3 seconds of CPU (still for 25 collections)
• With gathering smaps (not using smaps_rollup), it used 88.9 seconds of CPU (still for 25 collections)

Note that the last case should occur with older Linux kernel that don't implement smaps_rollup (before 4.15.x).
I would say since the toggle is implemented, let's keep it.

[ncabatoff]

This looks great, thanks! Please merge in master, then we can merge this and I'll build a new release.

[PierreF]

Rebased (cherry-picked) the commit on master and fixed the two comment. Hope it's this way you though the options struct.

[ncabatoff]

Thanks again @PierreF!

[ncabatoff]

I can't build a release yet because some target platforms aren't supported by the tip of procfs (see prometheus/procfs#294, https://app.circleci.com/pipelines/github/ncabatoff/process-exporter/8/workflows/97bdbcfe-2b97-464e-b846-60d327641e2f/jobs/57) but I'll keep an eye on it and hopefully have one out this week.