Generator Support for CounterBasedGauge64

[craized]

I have a Metaswitch MIB that imports CounterBasedGauge64 from HCNUM-TC. When I use the generator, they get parsed as counters instead of gauges.

Thanks for any consideration in adding support for this data type.

After a naive code review, I imagine the fix would just be adding it to the list below. https://github.com/prometheus/snmp_exporter/blob/0caff5465662870282e8637b68ad52189a40933d/generator/tree.go#L151-L157