Overhaul stats: add `TYPE` and `HELP` to Prometheus export

[josecelano]

More info: https://prometheus.io/docs/instrumenting/exposition_formats/#text-format-example

For example, for this line:

udp_tracker_server_requests_received_total{server_binding_ip="0.0.0.0",server_binding_port="6868",server_binding_protocol="udp"} 36661

We should add:

# HELP udp_tracker_server_requests_received_total Total number of UDP requests received
# TYPE udp_tracker_server_requests_received_total counter
udp_tracker_server_requests_received_total{server_binding_ip="0.0.0.0",server_binding_port="6868",server_binding_protocol="udp"} 36661

TYPE and HELP values can be obtained from the metric description:

metrics.metric_collection.describe_counter(
    &metric_name!(UDP_TRACKER_SERVER_REQUESTS_RECEIVED_TOTAL),
    Some(Unit::Count),
    Some(&MetricDescription::new("Total number of UDP requests received")),
);