extraHosts param should allow choosing local-ipv4

The extraHost ipAddress param should allow some sort of variable "${local-ipv4}" or some such way to choose the ecs servers own IP rather than a hard coded ip.

This would be to support applications not yet migrated into service discovery.

For example it would be easier to migrate hundreds of apps that use statsd to horizontally scale to use their own statsd on an ECS server than to get hundreds of repos updated.

This model:
ECS server -> local statsd -> graph system
Over this one:
ECS server -> UDP packets to some pet server -> graph system

Ideally applications use service discovery, but this is not done quickly in some places.

No repository file, test, or implementation entry point is named; start by reviewing ECS extraHost ipAddress behavior and the local statsd migration scenario described in the issue. Done would require a decided and documented way to resolve the ECS server’s local IPv4 address for extraHost, along with implementation details and validation criteria.

Written by the indexing model from the issue text.