Combine memory statistics into a single metric with labels

[robertomiranda]

What

Combine memory statistics into a single metric with labels. Additionally, now we are tracking PSS, which provides a more accurate representation of how much physical memory is uniquely and proportionally used by a process, especially in systems where many processes share memory.

Why

However, a single metric is enough to track the three key metrics for each worker: RSS, PSS, and Shared Memory. Pitchfork utilizes /proc/[pid]/smaps_rollup to report these values.

Anything else

/proc/[pid]/smaps_rollup reports its metrics to kilobytes, I'm adding bytes sufix to the metric name

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (6442f72) to head (4669448).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #70   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           38        38           
  Lines          854       852    -2     
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- Hits           854       852    -2     

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