google grpc: allow custom prefix paths for grpc

[AntonKanug]

Use-case:
When there is a L7 proxy in between multiple control planes, the proxy cannot distinguish which control plane the xds grpc is for by path.

Allow configuring custom prefixes for envoy google grpc for xds, so that following can be produced:

cluster.com/my-control-plane-1/envoy.service.discovery.v3.AggregatedDiscoveryService
cluster.com/my-control-plane-2/envoy.service.discovery.v3.AggregatedDiscoveryService

I have done this in our fork and is using it internally but is looking to contribute this as a feature.

Can also follow up with go/java control plane support for this out of the box when configuring the xds server.

[AntonKanug]

cc @nezdolik

[RyanTheOptimist]

cc: @markdroth @yanavlasov

[markdroth]

I don't think this is a good idea. gRPC is not designed to allow the path to be arbitrarily changed like this, and doing so is going to cause all sorts of problems.

Why do the control planes need to be distinguished by path? Can't the control plane simply figure this out by looking at the authority header on the incoming RPC?

[AntonKanug]

Today we only do path based routing, iIl look into using the authority header