[fixit] Fix flakiness for test case ChannelzSamplerTest.SimpleTest by reducing the chance of port conflict (#30620)

* Fix flakiness for test case ChannelzSamplerTest.SimpleTest by reducing the chance of port conflict

* Update

test/cpp/util/BUILD

@@ -332,6 +332,7 @@ grpc_cc_binary(
     srcs = ["channelz_sampler.cc"],
     external_deps = [
         "absl/flags:flag",
+        "absl/strings",
     ],
     language = "c++",
     tags = [

test/cpp/util/channelz_sampler_test.cc

@@ -26,6 +26,7 @@
 #include <string>
 #include <thread>
 
+#include "absl/strings/str_cat.h"
 #include "gtest/gtest.h"
 
 #include <grpc/grpc.h>
@@ -44,6 +45,7 @@
 #include "src/core/lib/gpr/env.h"
 #include "src/cpp/server/channelz/channelz_service.h"
 #include "src/proto/grpc/testing/test.grpc.pb.h"
+#include "test/core/util/port.h"
 #include "test/core/util/test_config.h"
 #include "test/cpp/util/subprocess.h"
 #include "test/cpp/util/test_credentials_provider.h"
@@ -174,6 +176,9 @@ int main(int argc, char** argv) {
   } else {
     g_root = ".";
   }
+
+  /// ensures the target address is unique even if this test is run in parallel
+  server_address = absl::StrCat("0.0.0.0:", grpc_pick_unused_port_or_die());
   int ret = RUN_ALL_TESTS();
   return ret;
 }