xds: LRS handle response contains multiple cluster names (#3393)

- The LRS client will find known clusters from the response, and send the loads
   - The LRS client support only one cluster now, will be extended to support multiple

xds/internal/balancer/lrs/lrs.go

@@ -332,12 +332,19 @@ func (ls *lrsStore) ReportTo(ctx context.Context, cc *grpc.ClientConn, clusterNa
 			grpclog.Warningf("lrs: failed to convert report interval: %v", err)
 			continue
 		}
-		if len(first.Clusters) != 1 {
-			grpclog.Warningf("lrs: received multiple clusters %v, expect one cluster", first.Clusters)
-			continue
+		// The LRS client should join the clusters it knows with the cluster
+		// list from response, and send loads for them.
+		//
+		// But the LRS client now only supports one cluster. TODO: extend it to
+		// support multiple clusters.
+		var clusterFoundInResponse bool
+		for _, c := range first.Clusters {
+			if c == clusterName {
+				clusterFoundInResponse = true
+			}
 		}
-		if first.Clusters[0] != clusterName {
-			grpclog.Warningf("lrs: received cluster is unexpected. Got %v, want %v", first.Clusters[0], clusterName)
+		if !clusterFoundInResponse {
+			grpclog.Warningf("lrs: received clusters %v does not contain expected {%v}", first.Clusters, clusterName)
 			continue
 		}
 		if first.ReportEndpointGranularity {

xds/internal/balancer/lrs/lrs_test.go

@@ -367,7 +367,7 @@ func (lrss *lrsServer) StreamLoadStats(stream lrsgrpc.LoadReportingService_Strea
 		return status.Errorf(codes.FailedPrecondition, "unexpected req: %+v", req)
 	}
 	if err := stream.Send(&lrspb.LoadStatsResponse{
-		Clusters:              []string{testService},
+		Clusters:              []string{testService, "another-cluster"},
 		LoadReportingInterval: lrss.reportingInterval,
 	}); err != nil {
 		return err