fix client signal level bug, add client signal-to-noise ratio

[jfieber]

• The client signal was being reported as dBm when it is, in fact a derived "connection quality" expressed as a percentage. This changes the metric name to be clear and not cause confusion over the same metric showing different semantics over time.
• Add the client signal-to-noise ratio
• Address what may be the issue in #22. I'm not convinced falling back on Name in the absence of HostName is quite correct though. Maybe we should always use Name, and add the sometimes-there-sometimes-not HostName as a separate label? That would be more consistent with Omada's data model.
• Add a different taken on generating the metrics description table in the Readme.