fix: Improve typing

[tomtom103]

What does this do?

Improve typing support.

Why do we need it?

Some methods are not typed / do not follow the typing from prometheus_client.

[codecov]

Codecov Report

Merging #231 (5c64a06) into master (5f533f4) will increase coverage by 0.30%.
The diff coverage is 96.00%.

@@            Coverage Diff             @@
##           master     #231      +/-   ##
==========================================
+ Coverage   94.98%   95.29%   +0.30%     
==========================================
  Files           5        5              
  Lines         339      340       +1     
==========================================
+ Hits          322      324       +2     
+ Misses         17       16       -1     

Impacted Files	Coverage Δ
...rc/prometheus_fastapi_instrumentator/middleware.py	96.62% <92.85%> (+1.12%)	⬆️
...ometheus_fastapi_instrumentator/instrumentation.py	97.10% <100.00%> (ø)
src/prometheus_fastapi_instrumentator/metrics.py	93.70% <100.00%> (ø)
src/prometheus_fastapi_instrumentator/routing.py	94.44% <100.00%> (+0.15%)	⬆️

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[trallnag]

Thanks

[trallnag]

https://github.com/trallnag/prometheus-fastapi-instrumentator/releases/tag/v5.11.1