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/gauge.go

@@ -0,0 +1,120 @@
+package metrics
+
+import "sync/atomic"
+
+// Gauges hold an int64 value that can be set arbitrarily.
+type Gauge interface {
+	Snapshot() Gauge
+	Update(int64)
+	Value() int64
+}
+
+// GetOrRegisterGauge returns an existing Gauge or constructs and registers a
+// new StandardGauge.
+func GetOrRegisterGauge(name string, r Registry) Gauge {
+	if nil == r {
+		r = DefaultRegistry
+	}
+	return r.GetOrRegister(name, NewGauge).(Gauge)
+}
+
+// NewGauge constructs a new StandardGauge.
+func NewGauge() Gauge {
+	if !Enabled {
+		return NilGauge{}
+	}
+	return &StandardGauge{0}
+}
+
+// NewRegisteredGauge constructs and registers a new StandardGauge.
+func NewRegisteredGauge(name string, r Registry) Gauge {
+	c := NewGauge()
+	if nil == r {
+		r = DefaultRegistry
+	}
+	r.Register(name, c)
+	return c
+}
+
+// NewFunctionalGauge constructs a new FunctionalGauge.
+func NewFunctionalGauge(f func() int64) Gauge {
+	if !Enabled {
+		return NilGauge{}
+	}
+	return &FunctionalGauge{value: f}
+}
+
+// NewRegisteredFunctionalGauge constructs and registers a new StandardGauge.
+func NewRegisteredFunctionalGauge(name string, r Registry, f func() int64) Gauge {
+	c := NewFunctionalGauge(f)
+	if nil == r {
+		r = DefaultRegistry
+	}
+	r.Register(name, c)
+	return c
+}
+
+// GaugeSnapshot is a read-only copy of another Gauge.
+type GaugeSnapshot int64
+
+// Snapshot returns the snapshot.
+func (g GaugeSnapshot) Snapshot() Gauge { return g }
+
+// Update panics.
+func (GaugeSnapshot) Update(int64) {
+	panic("Update called on a GaugeSnapshot")
+}
+
+// Value returns the value at the time the snapshot was taken.
+func (g GaugeSnapshot) Value() int64 { return int64(g) }
+
+// NilGauge is a no-op Gauge.
+type NilGauge struct{}
+
+// Snapshot is a no-op.
+func (NilGauge) Snapshot() Gauge { return NilGauge{} }
+
+// Update is a no-op.
+func (NilGauge) Update(v int64) {}
+
+// Value is a no-op.
+func (NilGauge) Value() int64 { return 0 }
+
+// StandardGauge is the standard implementation of a Gauge and uses the
+// sync/atomic package to manage a single int64 value.
+type StandardGauge struct {
+	value int64
+}
+
+// Snapshot returns a read-only copy of the gauge.
+func (g *StandardGauge) Snapshot() Gauge {
+	return GaugeSnapshot(g.Value())
+}
+
+// Update updates the gauge's value.
+func (g *StandardGauge) Update(v int64) {
+	atomic.StoreInt64(&g.value, v)
+}
+
+// Value returns the gauge's current value.
+func (g *StandardGauge) Value() int64 {
+	return atomic.LoadInt64(&g.value)
+}
+
+// FunctionalGauge returns value from given function
+type FunctionalGauge struct {
+	value func() int64
+}
+
+// Value returns the gauge's current value.
+func (g FunctionalGauge) Value() int64 {
+	return g.value()
+}
+
+// Snapshot returns the snapshot.
+func (g FunctionalGauge) Snapshot() Gauge { return GaugeSnapshot(g.Value()) }
+
+// Update panics.
+func (FunctionalGauge) Update(int64) {
+	panic("Update called on a FunctionalGauge")
+}