Add middlewares and one specific to expose /metrics endpoint for prometheus

[aguschin]

did this in spare time instead of #591

using https://github.com/trallnag/prometheus-fastapi-instrumentator

these middlewares can be enabled via CLI if referenced by their type ClassVar:

$ mlem --tb serve fastapi --model ../emoji/lyrics2emoji --middlewares.0 prometheus_fastapi

[codecov]

Codecov Report

Patch coverage: 88.52% and project coverage change: +0.01 🎉

Comparison is base (50f6d57) 85.97% compared to head (a39f74d) 85.99%.

❗ Current head a39f74d differs from pull request most recent head a01c051. Consider uploading reports for the commit a01c051 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #629      +/-   ##
==========================================
+ Coverage   85.97%   85.99%   +0.01%     
==========================================
  Files         107      109       +2     
  Lines        9770     9831      +61     
==========================================
+ Hits         8400     8454      +54     
- Misses       1370     1377       +7     

Impacted Files	Coverage Δ
mlem/contrib/sagemaker/runtime.py	80.64% <ø> (ø)
mlem/ext.py	88.28% <ø> (ø)
mlem/contrib/fastapi.py	83.91% <73.33%> (-1.25%)	⬇️
mlem/runtime/middleware.py	90.90% <90.90%> (ø)
mlem/contrib/prometheus.py	95.23% <95.23%> (ø)
mlem/runtime/server.py	89.22% <100.00%> (+0.19%)	⬆️

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