[network metrics] instance network interface schema

This is related to the ongoing work in [plumbing instance/guest
metrics](oxidecomputer/propolis#742) through to an
oximeter producer in propolis.

Includes:
- Rewriting the comment for `trait Target` in Oximeter to not being prepped for deprecation,
  as it does come in handy for custom needs
- The main differentiating field among instance network information here is `interface_id`, which is
  exposed to propolis via the client's `NetworkInterfaceRequest`. We currently match the `nic.slot` to the `port.slot` to pass which of the
  requested nics `interface_id` uuids are part of the request.