fix/feat: register metrics only once and add hit-miss

[Krukov]

Fix for #200

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 93.74%. Comparing base (f5179d2) to head (4a0ab1e).
Report is 2 commits behind head on master.

Files	Patch %	Lines
cashews/contrib/prometheus.py	73.33%	2 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   93.76%   93.74%   -0.03%     
==========================================
  Files          54       54              
  Lines        3529     3533       +4     
  Branches      609      610       +1     
==========================================
+ Hits         3309     3312       +3     
+ Misses        148      147       -1     
- Partials       72       74       +2     

Flag	Coverage Δ
unittests	93.65% <73.33%> (-0.03%)	⬇️

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