grpc.lb.backend_service is always unset in subchannel metrics

[ejona86]

https://github.com/grpc/proposal/blob/master/A94-subchannel-otel-metrics.md :

Since currently, only xDS is using these labels, the attributes will be set for each endpoint or address by cds (post-A74) or xds_cluster_resolver (pre-A74) LB policies.

cds vs cluster_resolver isn't important; cluster_resolver was merged into cds for A74, so all the code is going to the same place in either case.

I see the code setting locality, but nothing for backend_service:

grpc-java/xds/src/main/java/io/grpc/xds/ClusterResolverLoadBalancer.java

Lines 194 to 201 in 096c4d9

	endpoint.eag().getAttributes().toBuilder()
	.set(io.grpc.xds.XdsAttributes.ATTR_LOCALITY, locality)
	.set(EquivalentAddressGroup.ATTR_LOCALITY_NAME, localityName)
	.set(io.grpc.xds.XdsAttributes.ATTR_LOCALITY_WEIGHT,
	localityLbInfo.localityWeight())
	.set(io.grpc.xds.XdsAttributes.ATTR_SERVER_WEIGHT, weight)
	.set(XdsInternalAttributes.ATTR_ADDRESS_NAME, endpoint.hostname())
	.build();

I see the code trying to get the backend_service, but it is using a ResolutionResultAttr attribute, which should never be set in the EquivalentAddressGroup:

grpc-java/core/src/main/java/io/grpc/internal/InternalSubchannel.java

Line 607 in 096c4d9

addressIndex.getCurrentEagAttributes(), NameResolver.ATTR_BACKEND_SERVICE),

That attribute is being set today, but it is only in the NameResolver attributes, not in the EAG:

grpc-java/xds/src/main/java/io/grpc/xds/ClusterImplLoadBalancer.java

Lines 156 to 162 in 096c4d9

	childSwitchLb.handleResolvedAddresses(
	resolvedAddresses.toBuilder()
	.setAttributes(attributes.toBuilder()
	.set(NameResolver.ATTR_BACKEND_SERVICE, cluster)
	.build())
	.setLoadBalancingPolicyConfig(config.childConfig)
	.build());

(Note that code will be moving to cds/cluster_resolver to complete A75 in #12431. After that issue and this issue are fixed, all setting of backend_service would be within cds/cluster_resolver.)