Add option to group similar status codes

[claws]

Update ASGI middleware to support an option that will group similar status codes used by the status_codes_counter default metric. It will now support reporting "2xx", "3xx", etc instead of say explicit 200, 201, etc.

• Add unit tests
• Update user guide to note that developers using aioprometheus in unit tests should be familiar with calling REGISTRY.clear() between unit tests to return the default aioprometheus registry to a default state.
• Update change log.

[codecov-commenter]

Codecov Report

Merging #62 (eeacc09) into master (1d84221) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   95.48%   95.50%   +0.01%     
==========================================
  Files          17       17              
  Lines         776      778       +2     
  Branches       99       99              
==========================================
+ Hits          741      743       +2     
  Misses         19       19              
  Partials       16       16              

Impacted Files	Coverage Δ
src/aioprometheus/asgi/middleware.py	87.30% <100.00%> (+0.41%)	:arrow_up:

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