Middleware const_labels argument

[ngerakines]

This minor addition to the Middleware class allows const_labels to be provided and passed to the counters created at construction.

Usage:

app = FastAPI()
app.add_middleware(MetricsMiddleware, const_labels={"app": "webapi"})

[claws]

Looks like it's just a style issue holding up the CI pipeline. Can you run make style, see: style, then push the changes to this branch?

[codecov-commenter]

Codecov Report

Merging #73 (d11f2c6) into master (b07f21c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #73   +/-   ##
=======================================
  Coverage   95.50%   95.50%           
=======================================
  Files          17       17           
  Lines         778      779    +1     
  Branches      114      114           
=======================================
+ Hits          743      744    +1     
  Misses         19       19           
  Partials       16       16           

Impacted Files	Coverage Δ
src/aioprometheus/asgi/middleware.py	87.50% <100.00%> (+0.19%)	⬆️

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[claws]

Thanks