Code review comment for ~sajoupa/charm-telegraf:expose_ip_ranges

Thanks for the review.
We can do about anything with exta_options, but that's not ideal. Also, if we set an IP range manually with extra_options, then to allow related prometheuses, we must manually add their IP addresses to extra_options (and remove them on relation-departed, etc.).

This change also allows setting prometheus_output_port (and prometheus_ip_range) even when related to prometheus. Previously in a relation with prometheus, the charm could only set the port to 9126. https://github.com/prometheus/prometheus/wiki/Default-port-allocations says 9216 is "New Relic exporter", according to IANA it's bacula-sd so I made it default to 9103 (admittedly, changing the default could be in a separate MP).