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

For #56857

Change-Id: I58187d7c4112b35951014ab14f2969bed7f4c8e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/497319
Run-TryBot: Felix Geisendörfer <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
Reviewed-by: David Chase <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: felixge

src/runtime/metrics.go

@@ -300,6 +300,12 @@ func initMetrics() {
 				hist.counts[len(hist.counts)-1] = memstats.gcPauseDist.overflow.Load()
 			},
 		},
+		"/gc/scan/stack:bytes": {
+			compute: func(in *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = uint64(gcController.lastStackScan.Load())
+			},
+		},
 		"/gc/stack/starting-size:bytes": {
 			compute: func(in *statAggregate, out *metricValue) {
 				out.kind = metricKindUint64

src/runtime/metrics/description.go

@@ -294,6 +294,11 @@ var allDesc = []Description{
 		Kind:        KindFloat64Histogram,
 		Cumulative:  true,
 	},
+	{
+		Name:        "/gc/scan/stack:bytes",
+		Description: "The number of bytes of stack that were scanned last GC cycle.",
+		Kind:        KindUint64,
+	},
 	{
 		Name:        "/gc/stack/starting-size:bytes",
 		Description: "The stack size of new goroutines.",

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/stack:bytes
+		The number of bytes of stack that were scanned last GC cycle.
+
 	/gc/stack/starting-size:bytes
 		The stack size of new goroutines.