Add go1.26 plots cleanup+finalizer queues (#143)

* internal/plot: add plots GC finalizers and cleanup queue

* internal/plot: check described metrics exist

Author: arl

internal/plot/plot_gc_cleanups.go

@@ -0,0 +1,40 @@
+//go:build go1.26
+
+package plot
+
+import (
+	"runtime/metrics"
+	"time"
+)
+
+var _ = register(description{
+	metrics: []string{
+		"/gc/cleanups/executed:cleanups",
+		"/gc/cleanups/queued:cleanups",
+	},
+	getvalues: func() getvalues {
+		return func(_ time.Time, samples []metrics.Sample) any {
+			executed := samples[idx_gc_cleanups_executed_cleanups].Value.Uint64()
+			queued := samples[idx_gc_cleanups_queued_cleanups].Value.Uint64()
+			return []uint64{queued - executed}
+		}
+	},
+	layout: Scatter{
+		Name:  "gc-cleanups",
+		Tags:  []tag{tagGC},
+		Title: "GC Cleanups Queue",
+		Type:  "bar",
+		Layout: ScatterLayout{
+			BarMode: "stack",
+			Yaxis: ScatterYAxis{
+				Title: "cleanups",
+			},
+		},
+		Subplots: []Subplot{
+			{Unitfmt: "%{y}", Type: "bar", Name: "queue size"},
+		},
+		InfoText: `Approximate length of the cleanup functions queue (created by runtime.AddCleanup).
+Its <i>/gc/cleanups/queued:cleanups</i> - <i>/gc/cleanups/executed:cleanups</i>.
+Useful for detecting slow cleanups holding up the queue.
+`},
+})

internal/plot/plot_gc_finalizers.go

@@ -0,0 +1,40 @@
+//go:build go1.26
+
+package plot
+
+import (
+	"runtime/metrics"
+	"time"
+)
+
+var _ = register(description{
+	metrics: []string{
+		"/gc/finalizers/executed:finalizers",
+		"/gc/finalizers/queued:finalizers",
+	},
+	getvalues: func() getvalues {
+		return func(_ time.Time, samples []metrics.Sample) any {
+			executed := samples[idx_gc_finalizers_executed_finalizers].Value.Uint64()
+			queued := samples[idx_gc_finalizers_queued_finalizers].Value.Uint64()
+			return []uint64{queued - executed}
+		}
+	},
+	layout: Scatter{
+		Name:  "gc-finalizers",
+		Tags:  []tag{tagGC},
+		Title: "GC Finalizers Queue",
+		Type:  "bar",
+		Layout: ScatterLayout{
+			BarMode: "stack",
+			Yaxis: ScatterYAxis{
+				Title: "finalizers",
+			},
+		},
+		Subplots: []Subplot{
+			{Unitfmt: "%{y}", Type: "bar", Name: "queue size"},
+		},
+		InfoText: `Length of the finalizer functions queue (created by runtime.AddFinalizer).
+Its <i>/gc/finalizers/queued:cleanups</i> - <i>/gc/finalizers/executed:cleanups</i>.
+Useful for detecting finalizers overwhelming the queue, either by being too slow, or by there being too many of them.
+`},
+})

internal/plot/registry.go

@@ -84,6 +84,10 @@ func (r *registry) register(desc description) {
 	}
 
 	r.descriptions = append(r.descriptions, desc)
+
+	for _, metric := range desc.metrics {
+		r.mustidx(metric)
+	}
 }
 
 func mustidx(metric string) int {