[INTERNAL] Refactor macro for common use case

[kfdm]

Merge after #167

[codecov-io]

Codecov Report

Merging #168 into master will increase coverage by 0.02%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
+ Coverage   47.05%   47.07%   +0.02%     
==========================================
  Files          43       43              
  Lines        2425     2428       +3     
==========================================
+ Hits         1141     1143       +2     
- Misses       1284     1285       +1

Impacted Files	Coverage Δ
promgen/prometheus.py	60.92% <ø> (ø)	⬆️
promgen/views.py	49.85% <0%> (ø)	⬆️
promgen/templatetags/promgen.py	41% <75%> (+0.79%)	⬆️

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