AUTO DnsLookupFamily not working with Serverless VPC Connector

Hello!

I got the basic setup described by the [Endpoints for Cloud Run with ESPv2](https://cloud.google.com/endpoints/docs/openapi/get-started-cloud-run) tutorial:
- ESPv2 deployed in cloud run
- a cloud function

My goal is to make the function internal only which, from my understanding, requires that all egress traffic goes through a serverless VPC Connector.

When I add the VPC Connector though, I get 503 errors `upstream connect error or disconnect/reset before headers. reset reason: connection failure` (same as #170 ).

Just like #170 , I got it working by adding `--backend_dns_lookup_family=v4only` to ESPv2_ARGS. The function domain `....cloudfunctions.net` does support IPv6. Is that expected behavior?

Thanks!