fix(metrics): follow OTel conventions

[thomhurst]

Summary

• add singular UCUM annotation units and descriptions to every counter
• namespace system-specific attributes under kevlar.*
• emit circuit state values as lowercase snake_case
• lock the updated instrument and attribute schema in tests and documentation

This follows OpenTelemetry's general metric and naming guidance: product-specific namespaces, dot-delimited object properties, lowercase snake_case components, and singular annotated units for discrete counts.

Closes #59

Validation

• dotnet build Kevlar.slnx -c Release
• 483 unit, 16 integration, 19 analyzer, and 1 netstandard tests passed
• npm ci && npm run build

References

• https://opentelemetry.io/docs/specs/semconv/general/naming/
• https://opentelemetry.io/docs/specs/semconv/general/metrics/

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📥 Commits

Reviewing files that changed from the base of the PR and between 2342c5d and 2b54370.

📒 Files selected for processing (4)

• docs/docs/observability.md
• src/Kevlar/Internal/KevlarMetrics.cs
• src/Kevlar/KevlarDiagnostics.cs
• tests/Kevlar.Tests/MetricsTests.cs

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