[proposal] adding configurable status resolver on prometheus middleware

[Lee-Minjea]

related : #125

Add an errorToStatusCodeResolver option to the MiddlewareConfig that resolves error to status code.
Default behavior should remain as it is, referencing echo.HTTPError.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 63.77%. Comparing base (e3d5d99) to head (1e69e54).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
echoprometheus/prometheus.go	80.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
- Coverage   64.08%   63.77%   -0.32%     
==========================================
  Files           9        9              
  Lines         944      969      +25     
==========================================
+ Hits          605      618      +13     
- Misses        300      308       +8     
- Partials       39       43       +4     

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[aldas]

small remarks but overall I think this is nice addition.

[aldas]

LGTM