metrics: pull library and introduce ResettingTimer and InfluxDB reporter (#15910)

* go-metrics: fork library and introduce ResettingTimer and InfluxDB reporter.

* vendor: change nonsense/go-metrics to ethersphere/go-metrics

* go-metrics: add tests. move ResettingTimer logic from reporter to type.

* all, metrics: pull in metrics package in go-ethereum

* metrics/test: make sure metrics are enabled for tests

* metrics: apply gosimple rules

* metrics/exp, internal/debug: init expvar endpoint when starting pprof server

* internal/debug: tiny comment formatting fix

metrics/debug_test.go

@@ -0,0 +1,48 @@
+package metrics
+
+import (
+	"runtime"
+	"runtime/debug"
+	"testing"
+	"time"
+)
+
+func BenchmarkDebugGCStats(b *testing.B) {
+	r := NewRegistry()
+	RegisterDebugGCStats(r)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		CaptureDebugGCStatsOnce(r)
+	}
+}
+
+func TestDebugGCStatsBlocking(t *testing.T) {
+	if g := runtime.GOMAXPROCS(0); g < 2 {
+		t.Skipf("skipping TestDebugGCMemStatsBlocking with GOMAXPROCS=%d\n", g)
+		return
+	}
+	ch := make(chan int)
+	go testDebugGCStatsBlocking(ch)
+	var gcStats debug.GCStats
+	t0 := time.Now()
+	debug.ReadGCStats(&gcStats)
+	t1 := time.Now()
+	t.Log("i++ during debug.ReadGCStats:", <-ch)
+	go testDebugGCStatsBlocking(ch)
+	d := t1.Sub(t0)
+	t.Log(d)
+	time.Sleep(d)
+	t.Log("i++ during time.Sleep:", <-ch)
+}
+
+func testDebugGCStatsBlocking(ch chan int) {
+	i := 0
+	for {
+		select {
+		case ch <- i:
+			return
+		default:
+			i++
+		}
+	}
+}