deprecate system.mem.used.all metric

[aphralG]

Proposed changes

Deprecate system.mem.used.all metric and remove system.mem.used.all from tests
Fixed system.mem.used to have the correct value

Checklist

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• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (df8cf6e) 66.29% compared to head (c6a86eb) 66.09%.
Report is 2 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #485      +/-   ##
==========================================
- Coverage   66.29%   66.09%   -0.21%     
==========================================
  Files         118      118              
  Lines       13441    13439       -2     
==========================================
- Hits         8911     8882      -29     
- Misses       3936     3964      +28     
+ Partials      594      593       -1     

Files	Coverage Δ
...om/nginx/agent/v2/src/core/metrics/metrics_util.go	82.99% <ø> (-0.05%)	⬇️
...com/nginx/agent/v2/src/core/metrics/sources/mem.go	76.47% <100.00%> (-0.68%)	⬇️

... and 3 files with indirect coverage changes

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