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