Metrics with Prometheus (#2069)

* Basic process metrics

* Enable Prometheus by an environment variable

* Code formatting

* Documentation for Prometheus metrics

* Link from README to documentation of Prometheus

* Link from README to documentation of Prometheus

* Link from README to documentation of Prometheus

* Separate module for metrics + tests

* Metrics limited by IP

* Metrics are forbidded for all requets by default

* Code refactoring

* allowedIps passed as a string to PrometheusMetrics

* Handle missing config

* METRICS_PROMETHEUS_ALLOWED_IPS added to documentation

* Log info about enabled metrics

* Unused code removed

* package-lock.json updated

* prom-client updated to 11.1.2

* Code refactoring

* Do not read IP address from X-Forwarder-For header

README.md

@@ -132,12 +132,14 @@ SVG or JSON output. When deliberately changing the output, run
 `SNAPSHOT_DRY=1 npm run test:js:server` to preview changes to the saved
 snapshots, and `SNAPSHOT_UPDATE=1 npm run test:js:server` to update them.
 
-The server can be [configured][sentry configuration] to use [Sentry][sentry].
+The server can be configured to use [Sentry][] ([configuration][sentry configuration]) and [Prometheus][] ([configuration][prometheus configuration]).
 
 [package manager]: https://nodejs.org/en/download/package-manager/
 [snapshot tests]: https://glebbahmutov.com/blog/snapshot-testing/
-[sentry configuration]: doc/self-hosting.md#sentry
+[Prometheus]: https://prometheus.io/
+[prometheus configuration]: doc/self-hosting.md#prometheus
 [Sentry]: https://sentry.io/
+[sentry configuration]: doc/self-hosting.md#sentry
 
 Hosting your own server
 -----------------------