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

1 files changed, 68 insertions, 0 deletions

diff --git a/metrics/gauge_test.go b/metrics/gauge_test.go
new file mode 100644
index 000000000..1f2603d33
--- /dev/null
+++ b/metrics/gauge_test.go
@@ -0,0 +1,68 @@
+package metrics
+
+import (
+	"fmt"
+	"testing"
+)
+
+func BenchmarkGuage(b *testing.B) {
+	g := NewGauge()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		g.Update(int64(i))
+	}
+}
+
+func TestGauge(t *testing.T) {
+	g := NewGauge()
+	g.Update(int64(47))
+	if v := g.Value(); 47 != v {
+		t.Errorf("g.Value(): 47 != %v\n", v)
+	}
+}
+
+func TestGaugeSnapshot(t *testing.T) {
+	g := NewGauge()
+	g.Update(int64(47))
+	snapshot := g.Snapshot()
+	g.Update(int64(0))
+	if v := snapshot.Value(); 47 != v {
+		t.Errorf("g.Value(): 47 != %v\n", v)
+	}
+}
+
+func TestGetOrRegisterGauge(t *testing.T) {
+	r := NewRegistry()
+	NewRegisteredGauge("foo", r).Update(47)
+	if g := GetOrRegisterGauge("foo", r); 47 != g.Value() {
+		t.Fatal(g)
+	}
+}
+
+func TestFunctionalGauge(t *testing.T) {
+	var counter int64
+	fg := NewFunctionalGauge(func() int64 {
+		counter++
+		return counter
+	})
+	fg.Value()
+	fg.Value()
+	if counter != 2 {
+		t.Error("counter != 2")
+	}
+}
+
+func TestGetOrRegisterFunctionalGauge(t *testing.T) {
+	r := NewRegistry()
+	NewRegisteredFunctionalGauge("foo", r, func() int64 { return 47 })
+	if g := GetOrRegisterGauge("foo", r); 47 != g.Value() {
+		t.Fatal(g)
+	}
+}
+
+func ExampleGetOrRegisterGauge() {
+	m := "server.bytes_sent"
+	g := GetOrRegisterGauge(m, nil)
+	g.Update(47)
+	fmt.Println(g.Value()) // Output: 47
+}