runtime/metrics: fix /gc/scan/* metrics

In the existing implementation, all /gc/scan/* metrics are
always equal to 0 due to the dependency on gcStatDep not being
set. This leads to gcStatAggregate always containing zeros, and
always reporting 0 for those metrics.

Also, add a test to ensure that /gc/scan/* metrics are not empty.

Fixes #62477.

Change-Id: I67497347d50ed5c3ce1719a18714c062ec938cab
Reviewed-on: https://go-review.googlesource.com/c/go/+/525595
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Felix Geisendörfer <[email protected]>

Author: Gandem

src/runtime/metrics.go

@@ -190,24 +190,28 @@ func initMetrics() {
 			},
 		},
 		"/gc/scan/globals:bytes": {
+			deps: makeStatDepSet(gcStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {
 				out.kind = metricKindUint64
 				out.scalar = in.gcStats.globalsScan
 			},
 		},
 		"/gc/scan/heap:bytes": {
+			deps: makeStatDepSet(gcStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {
 				out.kind = metricKindUint64
 				out.scalar = in.gcStats.heapScan
 			},
 		},
 		"/gc/scan/stack:bytes": {
+			deps: makeStatDepSet(gcStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {
 				out.kind = metricKindUint64
 				out.scalar = in.gcStats.stackScan
 			},
 		},
 		"/gc/scan/total:bytes": {
+			deps: makeStatDepSet(gcStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {
 				out.kind = metricKindUint64
 				out.scalar = in.gcStats.totalScan
@@ -667,7 +671,7 @@ func (a *cpuStatsAggregate) compute() {
 	// a.cpuStats.accumulate(nanotime(), gcphase == _GCmark)
 }
 
-// cpuStatsAggregate represents various GC stats obtained from the runtime
+// gcStatsAggregate represents various GC stats obtained from the runtime
 // acquired together to avoid skew and inconsistencies.
 type gcStatsAggregate struct {
 	heapScan    uint64

src/runtime/metrics_test.go

@@ -405,6 +405,9 @@ func TestReadMetricsConsistency(t *testing.T) {
 	if gc.pauses < gc.numGC*2 {
 		t.Errorf("fewer pauses than expected: got %d, want at least %d", gc.pauses, gc.numGC*2)
 	}
+	if totalScan.got <= 0 {
+		t.Errorf("scannable GC space is empty: %d", totalScan.got)
+	}
 	if totalScan.got != totalScan.want {
 		t.Errorf("/gc/scan/total:bytes doesn't line up with sum of /gc/scan*: total %d vs. sum %d", totalScan.got, totalScan.want)
 	}