Add path, where metrics exposed, to config

[vbaranov]

We'd like to configure two metrics endpoints: one is unavailable from the outside and 2nd one is public. However, currently, seems, it is impossible to change the default path /metrics, where metrics are exposed.

[vbaranov]

Seems, feature is supported in https://hexdocs.pm/prometheus_plugs/Prometheus.PlugExporter. I am closing this.