fix(metrics): aggregate partition gauges

[thomhurst]

Summary

• aggregate concurrency and rate-limit gauge values that share a shield name and strategy index
• add kevlar.circuit_breaker.instances, grouped by bounded circuit state, for partition-safe breaker totals
• retain the per-instance numeric circuit-state compatibility gauge and document when to use each instrument

Motivation

Partitions intentionally omit their keys from metric tags, so stateful strategies in a PartitionedShield previously emitted colliding series. Exporters could keep an arbitrary last value. Aggregation now produces one low-cardinality series per identity, with breaker counts split by state.

Fixes #348

Validation

• dotnet build Kevlar.slnx -c Release (0 warnings, 0 errors)
• dotnet run --project tests/Kevlar.Tests -c Release -f net8.0 --no-build -- --timeout 5m (1227 passed)
• dotnet run --project tests/Kevlar.Tests -c Release -f net10.0 --no-build -- --timeout 5m (1260 passed)
• net10 coverage-mode unit suite (1260 passed; new aggregation iterators fully covered)
• pwsh scripts/Verify-ApiDocs.ps1
• pwsh scripts/Verify-Docs.ps1
• pwsh scripts/Build-ApiDocs.ps1 -SkipBuild (0 warnings, 0 errors)
• npm run build from docs/

Docs changes are text/table-only. In-app browser backend was unavailable for a screenshot; the Docusaurus production build passed.

[coderabbitai]

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