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/influxdb/README.md

@@ -0,0 +1,30 @@
+go-metrics-influxdb
+===================
+
+This is a reporter for the [go-metrics](https://github.com/rcrowley/go-metrics) library which will post the metrics to [InfluxDB](https://influxdb.com/).
+
+Note
+----
+
+This is only compatible with InfluxDB 0.9+.
+
+Usage
+-----
+
+```go
+import "github.com/vrischmann/go-metrics-influxdb"
+
+go influxdb.InfluxDB(
+    metrics.DefaultRegistry, // metrics registry
+    time.Second * 10,        // interval
+    "http://localhost:8086", // the InfluxDB url
+    "mydb",                  // your InfluxDB database
+    "myuser",                // your InfluxDB user
+    "mypassword",            // your InfluxDB password
+)
+```
+
+License
+-------
+
+go-metrics-influxdb is licensed under the MIT license. See the LICENSE file for details.