runtime/metrics: add the number of Go-to-C calls

For #47216.

Change-Id: I1c2cd518e6ff510cc3ac8d8f72fd52eadcabc16c
Reviewed-on: https://go-review.googlesource.com/c/go/+/404306
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>

src/runtime/metrics.go

@@ -65,6 +65,12 @@ func initMetrics() {
 
 	timeHistBuckets = timeHistogramMetricsBuckets()
 	metrics = map[string]metricData{
+		"/cgo/go-to-c-calls:calls": {
+			compute: func(_ *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = uint64(NumCgoCall())
+			},
+		},
 		"/gc/cycles/automatic:gc-cycles": {
 			deps: makeStatDepSet(sysStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {

src/runtime/metrics/description.go

@@ -51,6 +51,12 @@ type Description struct {
 // The English language descriptions below must be kept in sync with the
 // descriptions of each metric in doc.go.
 var allDesc = []Description{
+	{
+		Name:        "/cgo/go-to-c-calls:calls",
+		Description: "Count of calls made from Go to C by the current process.",
+		Kind:        KindUint64,
+		Cumulative:  true,
+	},
 	{
 		Name:        "/gc/cycles/automatic:gc-cycles",
 		Description: "Count of completed GC cycles generated by the Go runtime.",

src/runtime/metrics/doc.go

@@ -51,6 +51,9 @@ classes of floating-point values: NaN, infinity.
 
 Below is the full list of supported metrics, ordered lexicographically.
 
+	/cgo/go-to-c-calls:calls
+		Count of calls made from Go to C by the current process.
+
 	/gc/cycles/automatic:gc-cycles
 		Count of completed GC cycles generated by the Go runtime.
 

src/runtime/metrics_test.go

@@ -46,6 +46,8 @@ func TestReadMetrics(t *testing.T) {
 	var mallocs, frees uint64
 	for i := range samples {
 		switch name := samples[i].Name; name {
+		case "/cgo/go-to-c-calls:calls":
+			checkUint64(t, name, samples[i].Value.Uint64(), uint64(runtime.NumCgoCall()))
 		case "/memory/classes/heap/free:bytes":
 			checkUint64(t, name, samples[i].Value.Uint64(), mstats.HeapIdle-mstats.HeapReleased)
 		case "/memory/classes/heap/released:bytes":