IPv4 is preferred to IPv6 about bmc

[gebn]

This is intended behaviour by Go. The logic goes that, as Resolve*Addr() functions only return one address, they should prefer v4 on the assumption IPv6 would break v4-only users. A specific family can be forced by hardcoding address literals, which is probably a good strategy regardless, given the exporter uses long-lived connections. This just needs documenting.