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

For #56857

Change-Id: I10dbc5db506c95b7578c2b6baf051a351f68bb2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/497576
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

src/runtime/metrics.go

@@ -201,6 +201,12 @@ func initMetrics() {
 				out.scalar = gcController.heapScan.Load()
 			},
 		},
+		"/gc/scan/total:bytes": {
+			compute: func(in *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = gcController.globalsScan.Load() + gcController.heapScan.Load() + gcController.lastStackScan.Load()
+			},
+		},
 		"/gc/heap/allocs-by-size:bytes": {
 			deps: makeStatDepSet(heapStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {

src/runtime/metrics/description.go

@@ -309,6 +309,11 @@ var allDesc = []Description{
 		Description: "The number of bytes of stack that were scanned last GC cycle.",
 		Kind:        KindUint64,
 	},
+	{
+		Name:        "/gc/scan/total:bytes",
+		Description: "The total amount space that is scannable. Sum of all metrics in /gc/scan.",
+		Kind:        KindUint64,
+	},
 	{
 		Name:        "/gc/stack/starting-size:bytes",
 		Description: "The stack size of new goroutines.",

src/runtime/metrics/doc.go

@@ -225,6 +225,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 	/gc/scan/stack:bytes
 		The number of bytes of stack that were scanned last GC cycle.
 
+	/gc/scan/total:bytes
+		The total amount space that is scannable. Sum of all metrics in
+		/gc/scan.
+
 	/gc/stack/starting-size:bytes
 		The stack size of new goroutines.