fix: add validation not to accept negative values for monotonic counters

[taisho6339]

Fixes #3037

Changes

• Add clear documentation for monotonic counters, mentioning that negative values are dropped
• Add validations to reject negative values in sdk for Counter and ObservableCounter

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

❌ Patch coverage is 88.97638% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.8%. Comparing base (df412fe) to head (4adf9b8).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/internal/sum.rs	88.8%	8 Missing ⚠️
...emetry-sdk/src/metrics/internal/precomputed_sum.rs	89.0%	6 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #3260    +/-   ##
======================================
  Coverage   80.8%   80.8%            
======================================
  Files        129     129            
  Lines      23203   23330   +127     
======================================
+ Hits       18750   18863   +113     
- Misses      4453    4467    +14     

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