Bring back total_system_memory_bytes metric.

[juris]

As discussed in #228, added total_system_memory_bytes metric back.

[codecov]

Codecov Report

Merging #229 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #229   +/-   ##
=======================================
  Coverage   85.02%   85.02%           
=======================================
  Files           2        2           
  Lines         621      621           
=======================================
  Hits          528      528           
  Misses         67       67           
  Partials       26       26

Impacted Files	Coverage Δ
exporter/redis.go	87.9% <ø> (ø)	⬆️

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[coveralls]

Coverage remained the same at 88.204% when pulling 6d02d0a on juris:master into c50e853 on oliver006:master.

[oliver006]

👍

Thanks!