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 (53c9f47) to head (b48c4a2).

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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[cijothomas]

I am not sure if the idea of producing one log per invalid metric measurement is a good idea. For now, let's keep it to debug.

A better idea could be to have an Sdk internal metric, that'd track all measurement_drops... We can discuss this as a future improvement.

[cijothomas]

while its true that the spec says API should not validate this, and instead it is upto the implementations (i.e SDK) to do this validation, the SDK spec does not explicitly state what should it do when the value is negative. I'd love to get a spec clarification before implementing this.

I did take this route long ago (open-telemetry/opentelemetry-dotnet#2519) but never got the time to clarify if this is the right behavior to do in the sdk.