Values:
GOOGLE_SERVICE_TYPE_UNSPECIFIED (0):
Unspecified Google Service. Includes most of
Google APIs and services.
IAP (1):
Identity aware proxy.
https://cloud.google.com/iap/docs/using-tcp-forwarding
GFE_PROXY_OR_HEALTH_CHECK_PROBER (2):
One of two services sharing IP ranges:
- Load Balancer proxy
- Centralized Health Check prober
https://cloud.google.com/load-balancing/docs/firewall-rules
CLOUD_DNS (3):
Connectivity from Cloud DNS to forwarding
targets or alternate name servers that use
private routing.
https://cloud.google.com/dns/docs/zones/forwarding-zones#firewall-rules
https://cloud.google.com/dns/docs/policies#firewall-rules