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

vendor/github.com/influxdata/influxdb/models/statistic.go

@@ -0,0 +1,42 @@
+package models
+
+// Statistic is the representation of a statistic used by the monitoring service.
+type Statistic struct {
+	Name   string                 `json:"name"`
+	Tags   map[string]string      `json:"tags"`
+	Values map[string]interface{} `json:"values"`
+}
+
+// NewStatistic returns an initialized Statistic.
+func NewStatistic(name string) Statistic {
+	return Statistic{
+		Name:   name,
+		Tags:   make(map[string]string),
+		Values: make(map[string]interface{}),
+	}
+}
+
+// StatisticTags is a map that can be merged with others without causing
+// mutations to either map.
+type StatisticTags map[string]string
+
+// Merge creates a new map containing the merged contents of tags and t.
+// If both tags and the receiver map contain the same key, the value in tags
+// is used in the resulting map.
+//
+// Merge always returns a usable map.
+func (t StatisticTags) Merge(tags map[string]string) map[string]string {
+	// Add everything in tags to the result.
+	out := make(map[string]string, len(tags))
+	for k, v := range tags {
+		out[k] = v
+	}
+
+	// Only add values from t that don't appear in tags.
+	for k, v := range t {
+		if _, ok := tags[k]; !ok {
+			out[k] = v
+		}
+	}
+	return out
+}