Fix loading of prometheus server config

[einarmo]

Loading the _PromServerConfig configuration object failed as it was not annotated as a dataclass, so it had no default conversion from a dict, which caused the YAML deserialization to fail.

Turns out this has never been used.

[psalaberria002]

broken build, please fix

[finnag]

Also please add a more thourough description. What was wrong, how did you fix it, why did that work?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.35%. Comparing base (0f2aca9) to head (c5cc56d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #437      +/-   ##
==========================================
+ Coverage   77.32%   77.35%   +0.03%     
==========================================
  Files          42       42              
  Lines        3523     3524       +1     
==========================================
+ Hits         2724     2726       +2     
+ Misses        799      798       -1     

Files with missing lines	Coverage Δ
cognite/extractorutils/__init__.py	100.00% <100.00%> (ø)
cognite/extractorutils/configtools/elements.py	80.56% <100.00%> (+0.04%)	⬆️

... and 1 file with indirect coverage changes

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[psalaberria002]

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