Rename Beyla cache internal metrics to use attributes

[mariomac]

This will simplify some queries and dashboarding

[codecov]

Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.99%. Comparing base (dad9dab) to head (50ccb67).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/kubecache/instrument/imetrics.go	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1382      +/-   ##
==========================================
- Coverage   81.07%   80.99%   -0.08%     
==========================================
  Files         147      147              
  Lines       14770    14745      -25     
==========================================
- Hits        11975    11943      -32     
- Misses       2210     2217       +7     
  Partials      585      585              

Flag	Coverage Δ
integration-test	58.73% <0.00%> (+0.10%)	⬆️
k8s-integration-test	59.80% <88.23%> (-0.11%)	⬇️
oats-test	34.18% <0.00%> (+0.07%)	⬆️
unittests	51.89% <0.00%> (+0.02%)	⬆️

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