[metricbeat] first pass at adding vmstats to system/memory

[fearful-symmetry]

See #13175

This PR adds swap data from /proc/vmstat under linux to the system/memory metricset.

I wasn't sure how we wanted to add linux vmstat data to the memory metricset, so this is the most conservative implementation, which just adds swap data, in line with the original request. The vmstat data as a whole is quite large, so I didn't want to go ahead and just add everything.

We also had to update the go-sysinfo repo, hence why this PR is so large.

[jsoriano]

I have added some suggestions on field names, let me know what you think.

This will also need at least a changelog entry.

[fearful-symmetry]

Alright, pushed a change to to refactor the fields and make them a little more generic. I'm not sure about refactoring the VMStat call, since right now all the modifications we're making to the MapStr structure are happening in one place, and obscuring what we're doing to a different library entirely seems like a recipe for confusion later on.

[fearful-symmetry]

Bah, forgot to add a changelog entry