roundrobin: Delegate subchannel creation to pickfirst

[arjan-bal]

This PR switches round round robin to defer to pick first instead of creating and handling SubConns itself. This is part of Dualstack, and makes this a petiole. This PR makes roundrobin a wrapper around the endpoint sharding policy that implements the roundrobin logic.

RELEASE NOTES:

• balancer/roundrobin: Round Robin uses pick first child policies to manage endpoints instead of creating SubConns.
• balancer/roundrobin: Round Robin uses the Endpoints field instead of the Addresses field in resolver state updates.

[codecov]

Codecov Report

Attention: Patch coverage is 86.04651% with 6 lines in your changes missing coverage. Please review.

Project coverage is 81.95%. Comparing base (724f450) to head (b4a9944).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
balancer/roundrobin/roundrobin.go	86.36%	2 Missing and 1 partial ⚠️
internal/testutils/wrr.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7966      +/-   ##
==========================================
- Coverage   82.28%   81.95%   -0.34%     
==========================================
  Files         381      381              
  Lines       38539    38554      +15     
==========================================
- Hits        31712    31597     -115     
- Misses       5535     5641     +106     
- Partials     1292     1316      +24     

Files with missing lines	Coverage Δ
balancer/weightedroundrobin/balancer.go	82.92% <100.00%> (-0.79%)	⬇️
internal/testutils/balancer.go	85.56% <100.00%> (+3.21%)	⬆️
balancer/roundrobin/roundrobin.go	86.95% <86.36%> (-13.05%)	⬇️
internal/testutils/wrr.go	78.26% <0.00%> (-11.74%)	⬇️

... and 29 files with indirect coverage changes

[dfawley]

This was in the other PR, too.

Probably we should have endpointsharding produce a parsed PF config if it's going to be used frequently?

[arjan-bal]

That's a nice idea. I've raised a PR with this change: #8007

PTAL, I'll rebase both the PRs once #8007 is merged.

[dfawley]

This needed to be done eventually. But maybe for similar things in the future...it could have been easier for this PR (and may still be easier when dealing with future migration PRs) to make the manual resolver's UpdateState and InitialState methods set Endpoints according to the default behavior if it's unset.

[arjan-bal]

This change was not required since the code to populate endpoints based using addresses is present in resolver_wrapper.go:

grpc-go/resolver_wrapper.go

Lines 125 to 132 in f35fb34

	if s.Endpoints == nil {
	s.Endpoints = make([]resolver.Endpoint, 0, len(s.Addresses))
	for _, a := range s.Addresses {
	ep := resolver.Endpoint{Addresses: []resolver.Address{a}, Attributes: a.BalancerAttributes}
	ep.Addresses[0].BalancerAttributes = nil
	s.Endpoints = append(s.Endpoints, ep)
	}
	}

I've reverted changes to tests that use the manual resolver. In tests that call balancer.UpdateClientConnState directly, without creating a real channel, conversion from addresses to endpoints is required.

[dfawley]

This is still necessary, unfortunately, as only balancer.Balancer functions are transferred when embedding.