runtime/metrics: add gomaxprocs metric

For #47216.

Change-Id: Ib2d48c4583570a2dae9510a52d4c6ffc20161b31
Reviewed-on: https://go-review.googlesource.com/c/go/+/404305
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>

src/runtime/metrics.go

@@ -280,6 +280,12 @@ func initMetrics() {
 					in.sysStats.gcMiscSys + in.sysStats.otherSys
 			},
 		},
+		"/sched/gomaxprocs:threads": {
+			compute: func(_ *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = uint64(gomaxprocs)
+			},
+		},
 		"/sched/goroutines:goroutines": {
 			compute: func(_ *statAggregate, out *metricValue) {
 				out.kind = metricKindUint64

src/runtime/metrics/description.go

@@ -220,6 +220,11 @@ var allDesc = []Description{
 		Description: "All memory mapped by the Go runtime into the current process as read-write. Note that this does not include memory mapped by code called via cgo or via the syscall package. Sum of all metrics in /memory/classes.",
 		Kind:        KindUint64,
 	},
+	{
+		Name:        "/sched/gomaxprocs:threads",
+		Description: "The current runtime.GOMAXPROCS setting, or the number of operating system threads that can execute user-level Go code simultaneously.",
+		Kind:        KindUint64,
+	},
 	{
 		Name:        "/sched/goroutines:goroutines",
 		Description: "Count of live goroutines.",

src/runtime/metrics/doc.go

@@ -167,6 +167,11 @@ Below is the full list of supported metrics, ordered lexicographically.
 		by code called via cgo or via the syscall package.
 		Sum of all metrics in /memory/classes.
 
+	/sched/gomaxprocs:threads
+		The current runtime.GOMAXPROCS setting, or the number of
+		operating system threads that can execute user-level Go code
+		simultaneously.
+
 	/sched/goroutines:goroutines
 		Count of live goroutines.
 

src/runtime/metrics_test.go

@@ -223,6 +223,10 @@ func TestReadMetricsConsistency(t *testing.T) {
 			for i := range h.Counts {
 				gc.pauses += h.Counts[i]
 			}
+		case "/sched/gomaxprocs:threads":
+			if got, want := samples[i].Value.Uint64(), uint64(runtime.GOMAXPROCS(-1)); got != want {
+				t.Errorf("gomaxprocs doesn't match runtime.GOMAXPROCS: got %d, want %d", got, want)
+			}
 		case "/sched/goroutines:goroutines":
 			if samples[i].Value.Uint64() < 1 {
 				t.Error("number of goroutines is less than one")