Tor metrics Prometheus exporter

Setup a Tor metrics Prometheus exporter, so Onionprobe could have two distinct Prometheus exporters:

1. The default, existing one with Onionprobe metrics.
2. Another with the Tor process metrics.

Tasks:

• Add an Option to setup a MetricsPort and MetricsPortPolicy on the spawned Tor process, but disabled by default.
• Include it also on the Prometheus collection and make it available as a Grafana dashboard, but disabled by default.
• Add a warning/document somewhere that this setting should be used with care.
• Document how to enable the feature in the standalone monitoring node, including again the warnings about this setting.

Documentation: tpo/core/tor#40762