Fix Cache to use variable labels

[lucacome]

Proposed changes

Caches were not using the variable labels.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 59 lines in your changes missing coverage. Please review.

Project coverage is 3.90%. Comparing base (68df05b) to head (28ab7ca).

Files	Patch %	Lines
collector/nginx_plus.go	0.00%	59 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #780      +/-   ##
========================================
- Coverage   3.93%   3.90%   -0.04%     
========================================
  Files          5       5              
  Lines       1297    1307      +10     
========================================
  Hits          51      51              
- Misses      1246    1256      +10     

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