fix: Fix regression multi app Prometheus registry

[trallnag]

Fixes #219

[codecov]

Codecov Report

Merging #220 (286ca19) into master (a9e21ac) will increase coverage by 0.01%.
The diff coverage is 86.66%.

❗ Current head 286ca19 differs from pull request most recent head feeb044. Consider uploading reports for the commit feeb044 to get more accurate results

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   95.72%   95.73%   +0.01%     
==========================================
  Files           5        5              
  Lines         304      305       +1     
==========================================
+ Hits          291      292       +1     
  Misses         13       13              

Impacted Files	Coverage Δ
src/prometheus_fastapi_instrumentator/metrics.py	96.52% <84.61%> (ø)
...rc/prometheus_fastapi_instrumentator/middleware.py	95.29% <100.00%> (+0.05%)	⬆️

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[trallnag]

There are two changes:

• Inline instrumentation closure into try catch block to prevent NameError.
• Checking if instrumentation function is None in middleware.