Provide function for finding address of interface used in outgoing connections

[embs]

I'm working on instrumenting go services so they may make Zipkin aware of their location and found (through this SO answer) this way for fetching service IP.

I saw there's already a function for getting address given an interface: https://github.com/weaveworks/common/blob/955c13089be4f5ee948aeca47af832d23e4bc394/network/interface.go#L9

I guess services will almost (?) always make use of eth0 interface but it seems to me that getting the address without fixing it for a given interface is a more robust way of finding service IP.

If so, shall we provide this, for instance, GetOutboundIP function in weaveworks/commons?