Default prometheus exporter port for rtrtr

[wk]

Prometheus maintains a list of allocations of default ports for exporters, and rtrtr has been added to the list as port 9810.

This could be reflected as a recommendation or a default in rtrtr.conf.system-service, rtrtr.conf, Dockerfile, and README.md

[wk]

A mention here suggests there are plans to re-use the HTTP server for purposes beyond metrics. In such a configuration, the preference for a reserved prometheus metrics port vs. a generic port such as 8080 becomes less apparent.

[partim]

The built-in server can listen on multiple addresses, so maybe this is of value, anyway?