feat(metrics): add gatus_results_endpoint_success

[onedr0p]

Summary

Fixes: #969

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.

[onedr0p]

@TwiN here is the implementation with the README updated to help differentiate between gatus_results_total and gatus_results_endpoint_success

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.05%. Comparing base (3107336) to head (7ff083c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #970      +/-   ##
==========================================
+ Coverage   76.02%   76.05%   +0.03%     
==========================================
  Files          74       74              
  Lines        6706     6716      +10     
==========================================
+ Hits         5098     5108      +10     
  Misses       1306     1306              
  Partials      302      302              

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[TwiN]

@onedr0p Excellent work! Thank you for the contribution.