fix boundary sampler so it is true to given rate

[jamesmoessis]

Fixes #225

Solution works accurately on both amd64 and arm64 architectures.

Adds test to ensure number of sampling decisions falls within a given range.

Avoid float to int conversion which is platform dependent for high values (such as trace IDs)

[codefromthecrypt]

feel free to drop or edit my comment suggestions. figured we might as well while looking at it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.11%. Comparing base (87e3b85) to head (0124bb8).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #226      +/-   ##
==========================================
+ Coverage   66.83%   68.11%   +1.27%     
==========================================
  Files          36       43       +7     
  Lines        1770     2744     +974     
==========================================
+ Hits         1183     1869     +686     
- Misses        542      821     +279     
- Partials       45       54       +9     

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[codefromthecrypt]

security fail is unrelated to your change of course https://github.com/openzipkin/zipkin-go/actions/runs/13170134138?pr=226

[codefromthecrypt]

thanks for the help!