Improve Starlette integration, fixes #72

[claws]

This change improves Starlette integration by fixing #72.

The change involves delaying creation of the middleware metrics until a call is made through to the middleware to update a metric. This approached seemed like the best method for handling Starlette occasionally rebuilding the middleware stack - which, due to the middleware previously creating metrics in the __init__ method, was the cause of the exceptions being triggered.

This change also makes minor tweaks to some project files and bumps the version in preparation for a new release. The tweaks are:

• include change log in docs.
• streamline Makefile rules
• Minor updates to dev docs
• include setup.py in style checks
• typos and grammar fixes

[codecov-commenter]

Codecov Report

Merging #74 (5bb7dc8) into master (72d6bd2) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
+ Coverage   95.50%   95.54%   +0.03%     
==========================================
  Files          17       17              
  Lines         779      785       +6     
  Branches      114      115       +1     
==========================================
+ Hits          744      750       +6     
  Misses         19       19              
  Partials       16       16              

Impacted Files	Coverage Δ
src/aioprometheus/__init__.py	100.00% <100.00%> (ø)
src/aioprometheus/asgi/middleware.py	88.57% <100.00%> (+1.07%)	:arrow_up:

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