Google's HTTP(S) load balancer has cross-region load balancing. But that's not a useful case for me (and tons of people I'd argue) because I have servers in unmanaged groups that need low latency communication among them. So I need to load balance unmanaged instance groups across ZONES e.g. zone-a unmanaged group, zone-b unmanaged group, and zone-c unmanaged group. What am I missing? Is this VERY basic use case not supported by Google?
BTW, the servers are in unmanaged groups because they MUST have slightly different configurations. Please don't tell me it's time to move to AWS...sigh
Add instances from a single region to the instance groups of this HTTPS load balancer. The cross region example's backend service spans regions, yours does not have to.
Whether regional, zonal, or unmanaged instance groups are practical depends on what these "slightly different configurations" are.
For an equal number of instances in each zone: one regional managed instance group (MIG) in the desired region. Multiple instance templates are possible with additional regional MIGs, but instance counts are evenly spread across zones.
If you must have different number of instances or instance templates: several zonal MIGs in the same region.
If you must manage the lifecycle of instances and don't like auto healing or scaling, unmanaged instance groups.