fix: metric EaseProbe_ping_loss Add by mistake

[fangpsh]

• gauge metric should be SET value, not ADD.
• fix MaxRtt metric loss.

[fangpsh]

I have found the commit that caused this bug, maybe we should re-review this pull request：

https://github.com/megaease/easeprobe/pull/386/files#diff-2905b6c764234f33b4bf658a5c5e0fa0d82289c07f56455c6ed348e60c296c36L142

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.71%. Comparing base (eef3a25) to head (5460418).
Report is 34 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #533      +/-   ##
==========================================
- Coverage   98.80%   98.71%   -0.09%     
==========================================
  Files          85       85              
  Lines        5867     5858       -9     
==========================================
- Hits         5797     5783      -14     
- Misses         51       56       +5     
  Partials       19       19              

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[samanhappy]

Thanks for your contribution. These are indeed mistakes that were introduced in the previous PR.