kuberesolver support for grpc

[seankhliao]

Is your feature request related to a problem? Please describe.

getting gRPC to properly load balance on k8s is not a straightforward endeavor, the sticky connections easily result in unbalanced load in a 2 tier deployment.
#4274 #8000

Describe the solution you'd like

native k8s resolver support
https://github.com/sercand/kuberesolver

Describe alternatives you've considered
headless services, client side sidecars, etc

[jmacd]

I would like to propose that the OCB support an optional list of underscore imports to be imported by the generated main.go. The OTel Collector doesn't need to do anything to support alternative load balancers and resolvers (since this PR released in v0.84.0), since gRPC has a static registration mechanism. IOW, all main.go needs to do is

import _ "github.com/sercand/kuberesolver"

then use an address with kubernetes:// as a prefix. Likewise, to add a custom load balancer, underscore-import it and
then set balancer_name in a configuration and gRPC will do the rest. I suspect many users of OCB would benefit from an underscore_imports directive, and these are canonical examples why.

[seankhliao]

Note that kuberesolver is a grpc resolver for name resolution, not a balancer.
We'd need additional config for this i think?
I think the specific kuberesolver package also doesn't do automatic registration.