Code review comment for ~aluria/charm-prometheus-blackbox-exporter:rewrite

Added mostly minor comments, questions and nits.

The only thing of of substance I'm wondering about is the use of unit ports (.get_unit_open_ports()) -- aiui those would be used as reference points (probe targets)? If that is the case I'm wondering if these wouldn't be too dynamic. E.g. how are they going to be updated if services change or are removed?

Also wonder about the scope of the bb exporter here. Should the bb exporter measure network health or service health? If a service goes down the listening port would go down as well, triggering an alert even though the network might be healthy.