xds: Fix WRR test flake with TSAN

Fixes:
```
org.mockito.exceptions.verification.TooManyActualInvocations:
metricSink.addLongCounter(
    <custom argument matcher>,
    1L,
    [directaddress:///wrr-metrics],
    [, ]
);
Wanted 1 time:
-> at io.grpc.xds.WeightedRoundRobinLoadBalancerTest.metricWithRealChannel(WeightedRoundRobinLoadBalancerTest.java:1307)
But was 2 times:
-> at io.grpc.internal.MetricRecorderImpl.addLongCounter(MetricRecorderImpl.java:103)
-> at io.grpc.internal.MetricRecorderImpl.addLongCounter(MetricRecorderImpl.java:103)

	at io.grpc.xds.WeightedRoundRobinLoadBalancerTest.metricWithRealChannel(WeightedRoundRobinLoadBalancerTest.java:1307)
```

Author: ejona86

xds/src/test/java/io/grpc/xds/WeightedRoundRobinLoadBalancerTest.java

@@ -21,6 +21,7 @@
 import static org.mockito.AdditionalAnswers.delegatesTo;
 import static org.mockito.ArgumentMatchers.argThat;
 import static org.mockito.Mockito.any;
+import static org.mockito.Mockito.atLeast;
 import static org.mockito.Mockito.eq;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
@@ -1303,8 +1304,8 @@ public void metricWithRealChannel() throws Exception {
     assertThat(recorder.getError()).isNull();
 
     // Make sure at least one metric works. The other tests will make sure other metrics and the
-    // edge cases are working.
-    verify(metrics).addLongCounter(
+    // edge cases are working. Since this is racy, we just care it happened at least once.
+    verify(metrics, atLeast(1)).addLongCounter(
         argThat((instr) -> instr.getName().equals("grpc.lb.wrr.rr_fallback")),
         eq(1L),
         eq(Arrays.asList("directaddress:///wrr-metrics")),