Fix flakiness in xds_cluster_end2end_test: XdsTest/CdsTest.CircuitBreaking/V3 (#32027)

* Fix flakiness in xds_cluster_end2end_test: XdsTest/CdsTest.CircuitBreaking/V3

* update comment and add sleep in one more instance

test/cpp/end2end/xds/xds_cluster_end2end_test.cc

@@ -218,6 +218,12 @@ TEST_P(CdsTest, CircuitBreaking) {
                       "circuit breaker drop");
   // Cancel one RPC to allow another one through.
   rpcs[0].CancelRpc();
+  // Add a sleep here to ensure the RPC cancellation has completed correctly
+  // before trying the next RPC. There maybe a slight delay between return of
+  // CANCELLED RPC status and update of internal state tracking the number of
+  // concurrent active requests.
+  gpr_sleep_until(gpr_time_add(gpr_now(GPR_CLOCK_REALTIME),
+                               gpr_time_from_millis(1000, GPR_TIMESPAN)));
   CheckRpcSendOk(DEBUG_LOCATION);
   // Clean up.
   for (size_t i = 1; i < kMaxConcurrentRequests; ++i) {
@@ -258,6 +264,12 @@ TEST_P(CdsTest, CircuitBreakingMultipleChannelsShareCallCounter) {
                       "circuit breaker drop");
   // Cancel one RPC to allow another one through
   rpcs[0].CancelRpc();
+  // Add a sleep here to ensure the RPC cancellation has completed correctly
+  // before trying the next RPC. There maybe a slight delay between return of
+  // CANCELLED RPC status and update of internal state tracking the number of
+  // concurrent active requests.
+  gpr_sleep_until(gpr_time_add(gpr_now(GPR_CLOCK_REALTIME),
+                               gpr_time_from_millis(1000, GPR_TIMESPAN)));
   CheckRpcSendOk(DEBUG_LOCATION);
   // Clean up.
   for (size_t i = 1; i < kMaxConcurrentRequests; ++i) {