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/meter_test.go

@@ -0,0 +1,73 @@
+package metrics
+
+import (
+	"testing"
+	"time"
+)
+
+func BenchmarkMeter(b *testing.B) {
+	m := NewMeter()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		m.Mark(1)
+	}
+}
+
+func TestGetOrRegisterMeter(t *testing.T) {
+	r := NewRegistry()
+	NewRegisteredMeter("foo", r).Mark(47)
+	if m := GetOrRegisterMeter("foo", r); 47 != m.Count() {
+		t.Fatal(m)
+	}
+}
+
+func TestMeterDecay(t *testing.T) {
+	ma := meterArbiter{
+		ticker: time.NewTicker(time.Millisecond),
+		meters: make(map[*StandardMeter]struct{}),
+	}
+	m := newStandardMeter()
+	ma.meters[m] = struct{}{}
+	go ma.tick()
+	m.Mark(1)
+	rateMean := m.RateMean()
+	time.Sleep(100 * time.Millisecond)
+	if m.RateMean() >= rateMean {
+		t.Error("m.RateMean() didn't decrease")
+	}
+}
+
+func TestMeterNonzero(t *testing.T) {
+	m := NewMeter()
+	m.Mark(3)
+	if count := m.Count(); 3 != count {
+		t.Errorf("m.Count(): 3 != %v\n", count)
+	}
+}
+
+func TestMeterStop(t *testing.T) {
+	l := len(arbiter.meters)
+	m := NewMeter()
+	if len(arbiter.meters) != l+1 {
+		t.Errorf("arbiter.meters: %d != %d\n", l+1, len(arbiter.meters))
+	}
+	m.Stop()
+	if len(arbiter.meters) != l {
+		t.Errorf("arbiter.meters: %d != %d\n", l, len(arbiter.meters))
+	}
+}
+
+func TestMeterSnapshot(t *testing.T) {
+	m := NewMeter()
+	m.Mark(1)
+	if snapshot := m.Snapshot(); m.RateMean() != snapshot.RateMean() {
+		t.Fatal(snapshot)
+	}
+}
+
+func TestMeterZero(t *testing.T) {
+	m := NewMeter()
+	if count := m.Count(); 0 != count {
+		t.Errorf("m.Count(): 0 != %v\n", count)
+	}
+}