time: provide non-default metric for asynctimerchan

The original implementation could not support a non-default metric,
but now that it is set during package time, we can. Do that.


Change-Id: If7929b0afa5b0142d299718d9bd811c5a8d1cf93
Reviewed-on: https://go-review.googlesource.com/c/go/+/589058
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/godebugs/table.go

@@ -25,7 +25,7 @@ type Info struct {
 // Note: After adding entries to this table, update the list in doc/godebug.md as well.
 // (Otherwise the test in this package will fail.)
 var All = []Info{
-	{Name: "asynctimerchan", Package: "time", Changed: 23, Old: "1", Opaque: true},
+	{Name: "asynctimerchan", Package: "time", Changed: 23, Old: "1"},
 	{Name: "execerrdot", Package: "os/exec"},
 	{Name: "gocachehash", Package: "cmd/go"},
 	{Name: "gocachetest", Package: "cmd/go"},

src/runtime/metrics/doc.go

@@ -230,6 +230,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 	/gc/stack/starting-size:bytes
 		The stack size of new goroutines.
 
+	/godebug/non-default-behavior/asynctimerchan:events
+		The number of non-default behaviors executed by the time package
+		due to a non-default GODEBUG=asynctimerchan=... setting.
+
 	/godebug/non-default-behavior/execerrdot:events
 		The number of non-default behaviors executed by the os/exec
 		package due to a non-default GODEBUG=execerrdot=... setting.

src/time/sleep.go

@@ -23,6 +23,7 @@ func syncTimer(c chan Time) unsafe.Pointer {
 	// If asynctimerchan=1, we don't even tell the runtime
 	// about channel timers, so that we get the pre-Go 1.23 code paths.
 	if asynctimerchan.Value() == "1" {
+		asynctimerchan.IncNonDefault()
 		return nil
 	}