Convert string output from SNMP to float in Meinberg Syncbox agent

[BrianJKoopman]

Some of the output from the Syncbox SNMP interface is provided as strings, for instance the mean path delay, which is reported as "5.111 us". This is inconvenient as it's not easily plotted in Grafana. It'd be better to convert this within the agent before publishing to the OCS feed so that we get floats within InfluxDB/Grafana.

We can't mix types in InfluxDB, so we won't be able to use the same field name we're using without re-writing all of what's currently in InfluxDB. Doing that might be the cleanest solution for users who want to plot this, since if we change the field name, both will still persist within the database.

I suspect the units might change in this string output depending on the magnitude of the offset, so some care will be needed in the conversion.