runtime/metrics: add /gc/gogc:percent

For #56857

Change-Id: I7e7d2ea3e6ab59291a4cd867c680605ad75bd21f
Reviewed-on: https://go-review.googlesource.com/c/go/+/497317
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/runtime/metrics.go

@@ -254,6 +254,12 @@ func initMetrics() {
 				out.scalar = uint64(gcController.memoryLimit.Load())
 			},
 		},
+		"/gc/gogc:percent": {
+			compute: func(in *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = uint64(gcController.gcPercent.Load())
+			},
+		},
 		"/gc/heap/live:bytes": {
 			deps: makeStatDepSet(heapStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {

src/runtime/metrics/description.go

@@ -193,6 +193,13 @@ var allDesc = []Description{
 		Kind:        KindUint64,
 		Cumulative:  true,
 	},
+	{
+		Name: "/gc/gogc:percent",
+		Description: "Heap size target percentage configured by the user, otherwise 100. This " +
+			"value is set by the GOGC environment variable, and the runtime/debug.SetGCPercent " +
+			"function.",
+		Kind: KindUint64,
+	},
 	{
 		Name: "/gc/gomemlimit:bytes",
 		Description: "Go runtime memory limit configured by the user, otherwise " +

src/runtime/metrics/doc.go

@@ -147,6 +147,11 @@ Below is the full list of supported metrics, ordered lexicographically.
 	/gc/cycles/total:gc-cycles
 		Count of all completed GC cycles.
 
+	/gc/gogc:percent
+		Heap size target percentage configured by the user, otherwise
+		100. This value is set by the GOGC environment variable, and the
+		runtime/debug.SetGCPercent function.
+
 	/gc/gomemlimit:bytes
 		Go runtime memory limit configured by the user, otherwise
 		math.MaxInt64. This value is set by the GOMEMLIMIT environment

src/runtime/metrics_test.go

@@ -37,6 +37,11 @@ func TestReadMetrics(t *testing.T) {
 	oldLimit := debug.SetMemoryLimit(limit)
 	defer debug.SetMemoryLimit(oldLimit)
 
+	// Set an GC percent to check the metric for it
+	gcPercent := 99
+	oldGCPercent := debug.SetGCPercent(gcPercent)
+	defer debug.SetGCPercent(oldGCPercent)
+
 	// Tests whether readMetrics produces values aligning
 	// with ReadMemStats while the world is stopped.
 	var mstats runtime.MemStats
@@ -150,6 +155,8 @@ func TestReadMetrics(t *testing.T) {
 			checkUint64(t, name, samples[i].Value.Uint64(), mstats.HeapObjects)
 		case "/gc/heap/goal:bytes":
 			checkUint64(t, name, samples[i].Value.Uint64(), mstats.NextGC)
+		case "/gc/gogc:percent":
+			checkUint64(t, name, samples[i].Value.Uint64(), uint64(gcPercent))
 		case "/gc/cycles/automatic:gc-cycles":
 			checkUint64(t, name, samples[i].Value.Uint64(), uint64(mstats.NumGC-mstats.NumForcedGC))
 		case "/gc/cycles/forced:gc-cycles":