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/json_test.go

@@ -0,0 +1,28 @@
+package metrics
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+)
+
+func TestRegistryMarshallJSON(t *testing.T) {
+	b := &bytes.Buffer{}
+	enc := json.NewEncoder(b)
+	r := NewRegistry()
+	r.Register("counter", NewCounter())
+	enc.Encode(r)
+	if s := b.String(); "{\"counter\":{\"count\":0}}\n" != s {
+		t.Fatalf(s)
+	}
+}
+
+func TestRegistryWriteJSONOnce(t *testing.T) {
+	r := NewRegistry()
+	r.Register("counter", NewCounter())
+	b := &bytes.Buffer{}
+	WriteJSONOnce(r, b)
+	if s := b.String(); s != "{\"counter\":{\"count\":0}}\n" {
+		t.Fail()
+	}
+}