feat: Make add() accept args funcs

[trallnag]

No description provided.

[codecov]

Codecov Report

Merging #230 (dd6c4a2) into master (0cb465d) will increase coverage by 4.45%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
+ Coverage   90.53%   94.98%   +4.45%     
==========================================
  Files           5        5              
  Lines         338      339       +1     
==========================================
+ Hits          306      322      +16     
+ Misses         32       17      -15     

Impacted Files	Coverage Δ
...ometheus_fastapi_instrumentator/instrumentation.py	97.10% <100.00%> (+0.04%)	⬆️
src/prometheus_fastapi_instrumentator/metrics.py	93.70% <0.00%> (+10.48%)	⬆️

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