private static Cluster slowStartCluster(int windowMillis) {
        final Duration window = Duration.newBuilder()
                                        .setNanos((int) TimeUnit.MILLISECONDS.toNanos(windowMillis))
                                        .build();
        final SlowStartConfig slowStartConfig =
                SlowStartConfig.newBuilder()
                               .setSlowStartWindow(window)
                               .build();
        return XdsTestResources.createCluster(clusterName, 0)
                               .toBuilder()
                               .setLbPolicy(LbPolicy.ROUND_ROBIN)
                               .setRoundRobinLbConfig(RoundRobinLbConfig.newBuilder()
                                                                        .setSlowStartConfig(slowStartConfig))
                               .build();
    }
WeightedRandomLoadBalancer is random, so we just call selectNow
for a full iteration to consume all pending entries.
xds/src/test/java/com/linecorp/armeria/xds/client/endpoint/RampingUpTest.java