Allow NodeIp targets for ExtensionServices

[Steffen911]

What

The current ExtensionService interface allows routing of requests to a Kubernetes Service which may route it to any receiving pod within the cluster. For our opentelemetry tracing this implies that traces are sent to an arbitrary collector instance. Our collector instances apply a node tag to each span which produces a mismatch in our observability. Contour spans may be attributed to nodes that don't have any envoy proxies on them.

Instead of only accepting services, I would like an option on the ExtensionService to route spans to a port on the node that envoy runs on, e.g. using the NODE_IP that we can access using the K8s downward api.

[github-actions]

Hey @Steffen911! Thanks for opening your first issue. We appreciate your contribution and welcome you to our community! We are glad to have you here and to have your input on Contour. You can also join us on our mailing list and in our channel in the Kubernetes Slack Workspace