runtime/metrics: add /gc/scan/globals:bytes

For #56857

Change-Id: I748fd2a33ee76d9a83ea42f2ebf6d9edda243301
Reviewed-on: https://go-review.googlesource.com/c/go/+/497320
Run-TryBot: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/metrics.go

@@ -189,6 +189,12 @@ func initMetrics() {
 				out.scalar = in.sysStats.gcCyclesDone
 			},
 		},
+		"/gc/scan/globals:bytes": {
+			compute: func(in *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = gcController.globalsScan.Load()
+			},
+		},
 		"/gc/heap/allocs-by-size:bytes": {
 			deps: makeStatDepSet(heapStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {

src/runtime/metrics/description.go

@@ -294,6 +294,11 @@ var allDesc = []Description{
 		Kind:        KindFloat64Histogram,
 		Cumulative:  true,
 	},
+	{
+		Name:        "/gc/scan/globals:bytes",
+		Description: "The total amount of global variable space that is scannable.",
+		Kind:        KindUint64,
+	},
 	{
 		Name:        "/gc/scan/stack:bytes",
 		Description: "The number of bytes of stack that were scanned last GC cycle.",

src/runtime/metrics/doc.go

@@ -216,6 +216,9 @@ Below is the full list of supported metrics, ordered lexicographically.
 		Distribution of individual GC-related stop-the-world pause
 		latencies. Bucket counts increase monotonically.
 
+	/gc/scan/globals:bytes
+		The total amount of global variable space that is scannable.
+
 	/gc/scan/stack:bytes
 		The number of bytes of stack that were scanned last GC cycle.