[Metricbeat] Collect Working Set Size memory metric on Linux

[geekpete]

Describe the enhancement:

Add WSS (Working Set Size) to the linux process metricset in metricbeat.
This might require work to the underlying GoSigar library to add this capability.

Describe a specific use case for the enhancement or feature:

Provides more visibility/detail with memory usage on Linux, useful to know for capacity planning and scalability analysis of your processes.

Methods to collect WSS on Linux:
http://www.brendangregg.com/wss.html

Seems already present for Windows in the gosigar library but not for Linux:
https://github.com/elastic/gosigar/blob/master/sys/windows/syscall_windows.go#L119-L134
https://github.com/cloudfoundry/gosigar/search?q=WorkingSetSize&type=Code&utf8=%E2%9C%93

Loosely related:
#8345

[botelastic]

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