Support GRPC ServiceExport

[solmonk]

Currently GRPC target groups are only created if a Service is direct backendRef of GRPCRoute. It is not supported in case of ServiceExport, thus not available for ServiceImport either.

Since at the time of TG creation we are not aware of RouteType (Route may not even exist at this point), the ideal solution to create both HTTP/GRPC target groups for ServiceExport. Our key-value cache structure is making this really hard to achieve, because RouteType is not part of the key. Changing cache key is a big one - requires touching every part involving target groups, but seems like the only way to proceed.

[evgeniy-khatko]

Description

Hello, at Venmo we also need this capability. The high level requirements would be very similar to how HTTPRoute works, for example:

apiVersion: gateway.networking.k8s.io/v1alpha2
kind: GRPCRoute
metadata:
  name: grpc-service-import
spec:
  parentRefs:
    - name: gateway-name
      sectionName: https
  rules:
    - backendRefs:
        - name: grpc-service-01
          kind: Service
          port: 50051
          weight: 50
        - name: grpc-service-02
          kind: ServiceImport
          port: 50051
          weight: 50

Context

GRPCRoute already allows specifying ServiceImport-s as backendRef-s, but created HTTPTargetGoup-s are of type HTTP1. Tests also show that creating gRPC target groups independently and using them as targets in pre-created GRPCRoute works fine. So we assume cross-cluster gRPC support can be added with proper configuration and documentation changes?

Thank you for your suggestions. I have addressed this issue in #756.