xds: Move backend service label plumbing to CDS#12882
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A75 changed aggregate cluster handling so aggregate clusters are represented as priority children whose children are CDS policies. In that structure, A89 expects backend service metric plumbing to go through CDS instead of xds_cluster_impl. Set NameResolver.ATTR_BACKEND_SERVICE from CdsLoadBalancer2 for non-aggregate leaf clusters so WRR metrics can read the backend service. Also add the grpc.lb.backend_service pick details label from the leaf CDS picker wrapper. Do not add the aggregate root cluster as backend_service. Remove the old backend_service attribute and pick details label handling from ClusterImplLoadBalancer.
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Addresses #12431.
This moves backend service metric label plumbing from
xds_cluster_impltocds, matching the A75/A89 aggregate cluster behavior.Previously,
xds_cluster_impladded thegrpc.lb.backend_servicepick-details label and propagatedNameResolver.ATTR_BACKEND_SERVICEto its child policy. That works for simple clusters, but it is the wrong layer after A75 because aggregate clusters need the leaf CDS cluster name, not the aggregate root cluster name.This change makes CDS responsible for backend service plumbing:
grpc.lb.backend_servicepick-details label using the CDS cluster nameNameResolver.ATTR_BACKEND_SERVICEto its child policy, so WRR can consume the backend service contextxds_cluster_implno longer owns backend service pick-details label or child attribute propagationThe existing
InternalEquivalentAddressGroup.ATTR_BACKEND_SERVICEpath is left unchanged for endpoint/subchannel metrics.Tests cover:
NameResolver.ATTR_BACKEND_SERVICEgrpc.lb.backend_servicexds_cluster_implno longer adding backend service labels or attributesPickDetailsConsumer, matching the realPickSubchannelArgscontract