runtime,runtime/metrics: export goroutine count as a metric

For #37112.

Change-Id: I994dfe848605b95ef6aec24f53869e929247e987
Reviewed-on: https://go-review.googlesource.com/c/go/+/247049
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/metrics.go

@@ -214,6 +214,12 @@ func initMetrics() {
 					in.sysStats.gcMiscSys + in.sysStats.otherSys
 			},
 		},
+		"/sched/goroutines:goroutines": {
+			compute: func(_ *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = uint64(gcount())
+			},
+		},
 	}
 	metricsInit = true
 }

src/runtime/metrics/description.go

@@ -163,6 +163,11 @@ var allDesc = []Description{
 		Description: "All memory mapped by the Go runtime into the current process as read-write. Note that this does not include memory mapped by code called via cgo or via the syscall package. Sum of all metrics in /memory/classes.",
 		Kind:        KindUint64,
 	},
+	{
+		Name:        "/sched/goroutines:goroutines",
+		Description: "Count of live goroutines.",
+		Kind:        KindUint64,
+	},
 }
 
 // All returns a slice of containing metric descriptions for all supported metrics.

src/runtime/metrics/doc.go

@@ -123,5 +123,8 @@ Supported metrics
 		as read-write. Note that this does not include memory mapped
 		by code called via cgo or via the syscall package.
 		Sum of all metrics in /memory/classes.
+
+	/sched/goroutines:goroutines
+		Count of live goroutines.
 */
 package metrics

src/runtime/metrics_test.go

@@ -145,6 +145,10 @@ func TestReadMetricsConsistency(t *testing.T) {
 			for i := range h.Counts {
 				gc.pauses += h.Counts[i]
 			}
+		case "/sched/goroutines:goroutines":
+			if samples[i].Value.Uint64() < 1 {
+				t.Error("number of goroutines is less than one")
+			}
 		}
 	}
 	if totalVirtual.got != totalVirtual.want {