fix(metrics): track cardinality by label tuple

[saurabhhhcodes]

Summary\n- track unique label tuples per metric instead of counting every event\n- keep previously seen tuples recordable after the cardinality cap is reached\n- apply the guard after extracting the actual Prometheus label values for syscall, TCP, disk, scheduler, and FD metrics\n- update cardinality tests for repeated tuples, over-limit new tuples, and independent metric budgets\n\nFixes #117\n\n## Validation\n- go test ./internal/metrics\n- go test ./... (fails before completing due to existing unrelated build error: internal/chaos/cpu.go:95:14: undefined: unix.Nanosleep)\n- go vet ./... (same existing internal/chaos build error)

[github-actions]

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