internal/plot: add goroutines plot for !go1.26 (#145)

The various metrics which break down the goroutines by state have been
added in go1.26. Before that we only had `/sched/goroutines:goroutine`
which was removed in a precedent commit.

Author: arl

internal/plot/plot_goroutines_pre_go1.26.go

@@ -0,0 +1,36 @@
+//go:build !go1.26
+
+package plot
+
+import (
+	"runtime/metrics"
+	"time"
+)
+
+var _ = register(description{
+	metrics: []string{
+		"/sched/goroutines:goroutines",
+	},
+	getvalues: func() getvalues {
+		return func(_ time.Time, samples []metrics.Sample) any {
+			goroutines := samples[idx_sched_goroutines_goroutines].Value.Uint64()
+			return []uint64{goroutines}
+		}
+	},
+
+	layout: Scatter{
+		Name:  "goroutines",
+		Tags:  []tag{tagScheduler},
+		Title: "Goroutines",
+		Type:  "scatter",
+		Layout: ScatterLayout{
+			Yaxis: ScatterYAxis{
+				Title: "goroutines",
+			},
+		},
+		Subplots: []Subplot{
+			{Name: "goroutines", Unitfmt: "%{y}"},
+		},
+		InfoText: `<i>Goroutines</i> is <b>/sched/goroutines</b>, the count of live goroutines`,
+	},
+})