Move precomputed sum code to a new file

[utpilla]

Changes

• Move precomputed sum code to a new file
• This would also help keep the tests for sum and precomputed sum more manageable

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 98.01325% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.8%. Comparing base (20625f9) to head (05cd920).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...emetry-sdk/src/metrics/internal/precomputed_sum.rs	98.0%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2036   +/-   ##
=====================================
  Coverage   76.8%   76.8%           
=====================================
  Files        122     123    +1     
  Lines      20984   20984           
=====================================
  Hits       16134   16134           
  Misses      4850    4850           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.