fix(metrics): track unique cardinality label tuples

[saurabhhhcodes]

Summary

• track unique label tuples per metric instead of counting every event
• keep already-seen label tuples recordable after the cardinality limit is reached
• apply the guard to the actual labels used by syscall, TCP, disk I/O, scheduler, and FD metrics
• add regression coverage for new tuple rejection and existing tuple preservation

Fixes #117.

Validation

• env GOCACHE=/private/tmp/kerno-go-build-cache go test ./internal/metrics -count=1
• git diff --check

env GOCACHE=/private/tmp/kerno-go-build-cache go test ./... was also attempted, but this macOS sandbox blocks unrelated packages: internal/chaos fails to build on unix.Nanosleep, and internal/ai panics because httptest cannot bind a local port.

[github-actions]

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