Audit code path when GrpcLB is not in ClassPath

[ejona86]

We need to guarantee that if GRPCLB addresses are provided by the NameResolver, they won't make it to a LB instance other than GRPCLB, as other LB types will not properly ignore the GRPCLB addresses and try to route traffic to them and never realize all such RPCs will fail.

Today it seems AutoConfiguredLoadBalancerFactory will throw, which isn't probably appropriate because it will cause the channel to go into panic mode. Instead, it could choose a LB instance that fails all RPCs with a clear Status. Or it could consider stripping the LB addresses and continue like normal, passing the non-LB addresses to the PF/RR/etc. If there are no non-LB addresses, maybe it could still fail RPCs with a clear status saying gRPCLB is required for this target.

We should also add a comment to the GRPCLB Attribute Key noting that LB policies won't receive it.

[ejona86]

Oh, and we should coordinate with C to make sure we have similar logic here. I think Go has gRPCLB unconditionally, so wouldn't be impacted.

[zhangkun83]

I talked with @markdroth. We both agree that it's a configuration error that the service is configured with grpclb but the client is not loaded with the required library, and that the correct thing to do is to fail all RPCs.

[ejona86]

@zhangkun83, note that means that existing services wouldn't be able to enable gRPCLB, as it would break some existing clients.