Go proposal: Goroutine metrics

Go 1.26 introduces new runtime metrics for goroutine monitoring, including per-state goroutine counts (waiting, runnable, running, not-in-go) and active thread counts. These metrics help identify production issues like lock contention, syscall bottlenecks, and CPU saturation by tracking goroutine behavior through the runtime/metrics package. The counters enable observability systems to detect scheduler problems and performance regressions without requiring full tracing.