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, 237 insertions, 0 deletions

diff --git a/metrics/resetting_timer.go b/metrics/resetting_timer.go
new file mode 100644
index 000000000..57bcb3134
--- /dev/null
+++ b/metrics/resetting_timer.go
@@ -0,0 +1,237 @@
+package metrics
+
+import (
+	"math"
+	"sort"
+	"sync"
+	"time"
+)
+
+// Initial slice capacity for the values stored in a ResettingTimer
+const InitialResettingTimerSliceCap = 10
+
+// ResettingTimer is used for storing aggregated values for timers, which are reset on every flush interval.
+type ResettingTimer interface {
+	Values() []int64
+	Snapshot() ResettingTimer
+	Percentiles([]float64) []int64
+	Mean() float64
+	Time(func())
+	Update(time.Duration)
+	UpdateSince(time.Time)
+}
+
+// GetOrRegisterResettingTimer returns an existing ResettingTimer or constructs and registers a
+// new StandardResettingTimer.
+func GetOrRegisterResettingTimer(name string, r Registry) ResettingTimer {
+	if nil == r {
+		r = DefaultRegistry
+	}
+	return r.GetOrRegister(name, NewResettingTimer).(ResettingTimer)
+}
+
+// NewRegisteredResettingTimer constructs and registers a new StandardResettingTimer.
+func NewRegisteredResettingTimer(name string, r Registry) ResettingTimer {
+	c := NewResettingTimer()
+	if nil == r {
+		r = DefaultRegistry
+	}
+	r.Register(name, c)
+	return c
+}
+
+// NewResettingTimer constructs a new StandardResettingTimer
+func NewResettingTimer() ResettingTimer {
+	if !Enabled {
+		return NilResettingTimer{}
+	}
+	return &StandardResettingTimer{
+		values: make([]int64, 0, InitialResettingTimerSliceCap),
+	}
+}
+
+// NilResettingTimer is a no-op ResettingTimer.
+type NilResettingTimer struct {
+}
+
+// Values is a no-op.
+func (NilResettingTimer) Values() []int64 { return nil }
+
+// Snapshot is a no-op.
+func (NilResettingTimer) Snapshot() ResettingTimer { return NilResettingTimer{} }
+
+// Time is a no-op.
+func (NilResettingTimer) Time(func()) {}
+
+// Update is a no-op.
+func (NilResettingTimer) Update(time.Duration) {}
+
+// Percentiles panics.
+func (NilResettingTimer) Percentiles([]float64) []int64 {
+	panic("Percentiles called on a NilResettingTimer")
+}
+
+// Mean panics.
+func (NilResettingTimer) Mean() float64 {
+	panic("Mean called on a NilResettingTimer")
+}
+
+// UpdateSince is a no-op.
+func (NilResettingTimer) UpdateSince(time.Time) {}
+
+// StandardResettingTimer is the standard implementation of a ResettingTimer.
+// and Meter.
+type StandardResettingTimer struct {
+	values []int64
+	mutex  sync.Mutex
+}
+
+// Values returns a slice with all measurements.
+func (t *StandardResettingTimer) Values() []int64 {
+	return t.values
+}
+
+// Snapshot resets the timer and returns a read-only copy of its contents.
+func (t *StandardResettingTimer) Snapshot() ResettingTimer {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+	currentValues := t.values
+	t.values = make([]int64, 0, InitialResettingTimerSliceCap)
+
+	return &ResettingTimerSnapshot{
+		values: currentValues,
+	}
+}
+
+// Percentiles panics.
+func (t *StandardResettingTimer) Percentiles([]float64) []int64 {
+	panic("Percentiles called on a StandardResettingTimer")
+}
+
+// Mean panics.
+func (t *StandardResettingTimer) Mean() float64 {
+	panic("Mean called on a StandardResettingTimer")
+}
+
+// Record the duration of the execution of the given function.
+func (t *StandardResettingTimer) Time(f func()) {
+	ts := time.Now()
+	f()
+	t.Update(time.Since(ts))
+}
+
+// Record the duration of an event.
+func (t *StandardResettingTimer) Update(d time.Duration) {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+	t.values = append(t.values, int64(d))
+}
+
+// Record the duration of an event that started at a time and ends now.
+func (t *StandardResettingTimer) UpdateSince(ts time.Time) {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+	t.values = append(t.values, int64(time.Since(ts)))
+}
+
+// ResettingTimerSnapshot is a point-in-time copy of another ResettingTimer.
+type ResettingTimerSnapshot struct {
+	values              []int64
+	mean                float64
+	thresholdBoundaries []int64
+	calculated          bool
+}
+
+// Snapshot returns the snapshot.
+func (t *ResettingTimerSnapshot) Snapshot() ResettingTimer { return t }
+
+// Time panics.
+func (*ResettingTimerSnapshot) Time(func()) {
+	panic("Time called on a ResettingTimerSnapshot")
+}
+
+// Update panics.
+func (*ResettingTimerSnapshot) Update(time.Duration) {
+	panic("Update called on a ResettingTimerSnapshot")
+}
+
+// UpdateSince panics.
+func (*ResettingTimerSnapshot) UpdateSince(time.Time) {
+	panic("UpdateSince called on a ResettingTimerSnapshot")
+}
+
+// Values returns all values from snapshot.
+func (t *ResettingTimerSnapshot) Values() []int64 {
+	return t.values
+}
+
+// Percentiles returns the boundaries for the input percentiles.
+func (t *ResettingTimerSnapshot) Percentiles(percentiles []float64) []int64 {
+	t.calc(percentiles)
+
+	return t.thresholdBoundaries
+}
+
+// Mean returns the mean of the snapshotted values
+func (t *ResettingTimerSnapshot) Mean() float64 {
+	if !t.calculated {
+		t.calc([]float64{})
+	}
+
+	return t.mean
+}
+
+func (t *ResettingTimerSnapshot) calc(percentiles []float64) {
+	sort.Sort(Int64Slice(t.values))
+
+	count := len(t.values)
+	if count > 0 {
+		min := t.values[0]
+		max := t.values[count-1]
+
+		cumulativeValues := make([]int64, count)
+		cumulativeValues[0] = min
+		for i := 1; i < count; i++ {
+			cumulativeValues[i] = t.values[i] + cumulativeValues[i-1]
+		}
+
+		t.thresholdBoundaries = make([]int64, len(percentiles))
+
+		thresholdBoundary := max
+
+		for i, pct := range percentiles {
+			if count > 1 {
+				var abs float64
+				if pct >= 0 {
+					abs = pct
+				} else {
+					abs = 100 + pct
+				}
+				// poor man's math.Round(x):
+				// math.Floor(x + 0.5)
+				indexOfPerc := int(math.Floor(((abs / 100.0) * float64(count)) + 0.5))
+				if pct >= 0 {
+					indexOfPerc -= 1 // index offset=0
+				}
+				thresholdBoundary = t.values[indexOfPerc]
+			}
+
+			t.thresholdBoundaries[i] = thresholdBoundary
+		}
+
+		sum := cumulativeValues[count-1]
+		t.mean = float64(sum) / float64(count)
+	} else {
+		t.thresholdBoundaries = make([]int64, len(percentiles))
+		t.mean = 0
+	}
+
+	t.calculated = true
+}
+
+// Int64Slice attaches the methods of sort.Interface to []int64, sorting in increasing order.
+type Int64Slice []int64
+
+func (s Int64Slice) Len() int           { return len(s) }
+func (s Int64Slice) Less(i, j int) bool { return s[i] < s[j] }
+func (s Int64Slice) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }