fix: decoding prometheus remote write proto doesn't reset the value

[waynexia]

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Refer to a related PR or issue link (optional)

closes #3495

What's changed and what's your intention?

Clear::clear() of Sample used to be an empty impl and causes incorrect data.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 84.93%. Comparing base (e433396) to head (038bc4f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3505      +/-   ##
==========================================
- Coverage   85.34%   84.93%   -0.42%     
==========================================
  Files         904      904              
  Lines      150172   150170       -2     
==========================================
- Hits       128169   127541     -628     
- Misses      22003    22629     +626     

[v0y4g3r]

LGTM