[example] Add WAIT_FOR_READY example (grpc#35219)

examples/cpp/wait_for_ready/BUILD

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+# Copyright 2023 the gRPC authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+licenses(["notice"])
+
+cc_binary(
+    name = "greeter_callback_client",
+    srcs = ["greeter_callback_client.cc"],
+    defines = ["BAZEL_BUILD"],
+    deps = [
+        "//:grpc++",
+        "//examples/protos:helloworld_cc_grpc",
+        "@com_google_absl//absl/flags:flag",
+        "@com_google_absl//absl/flags:parse",
+    ],
+)

examples/cpp/wait_for_ready/README.md

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+# gRPC C++ Wait-For-Ready Example
+
+The Wait-For-Ready example builds on the
+[Hello World Example](https://github.com/grpc/grpc/tree/master/examples/cpp/helloworld)
+and changes the gRPC client and server to show how to set wait-for-ready.
+
+For more information on wait-for-ready in gRPC, please refer to
+[gRPC Wait For Ready Semantics](https://github.com/grpc/grpc/blob/master/doc/wait-for-ready.md).
+
+## Running the example
+
+First run the client -
+
+```
+$ tools/bazel run examples/cpp/wait_for_ready:greeter_callback_client
+```
+
+On running this, we'll see 10 RPCs failed due to "Connection refused" errors.
+These RPCs did not have WAIT_FOR_READY set, resulting in the RPCs not waiting
+for the channel to be connected.
+
+The next 10 RPCs have WAIT_FOR_READY set, so the client will be waiting for the
+channel to be ready before progressing.
+
+Now, on a separate terminal, run the server -
+
+```
+$ tools/bazel run examples/cpp/helloworld:greeter_callback_server
+```
+
+The client channel should now be able to connect to the server, and the RPCs
+should succeed.

examples/cpp/wait_for_ready/greeter_callback_client.cc

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+//
+//
+// Copyright 2023 gRPC authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//
+
+#include <condition_variable>
+#include <iostream>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <thread>
+
+#include "absl/flags/flag.h"
+#include "absl/flags/parse.h"
+
+#include <grpcpp/grpcpp.h>
+
+#ifdef BAZEL_BUILD
+#include "examples/protos/helloworld.grpc.pb.h"
+#else
+#include "helloworld.grpc.pb.h"
+#endif
+
+ABSL_FLAG(std::string, target, "localhost:50051", "Server address");
+
+using grpc::Channel;
+using grpc::ClientContext;
+using grpc::Status;
+using helloworld::Greeter;
+using helloworld::HelloReply;
+using helloworld::HelloRequest;
+
+class GreeterClient {
+ public:
+  GreeterClient(std::shared_ptr<Channel> channel)
+      : stub_(Greeter::NewStub(channel)) {}
+
+  // Assembles the client's payload, sends it and presents the response back
+  // from the server.
+  std::string SayHello(const std::string& user, bool wait_for_ready) {
+    HelloRequest request;
+    request.set_name(user);
+    HelloReply reply;
+    ClientContext context;
+    context.set_wait_for_ready(wait_for_ready);
+    std::mutex mu;
+    std::condition_variable cv;
+    bool done = false;
+    Status status;
+    stub_->async()->SayHello(&context, &request, &reply,
+                             [&mu, &cv, &done, &status](Status s) {
+                               status = std::move(s);
+                               std::lock_guard<std::mutex> lock(mu);
+                               done = true;
+                               cv.notify_one();
+                             });
+
+    std::unique_lock<std::mutex> lock(mu);
+    while (!done) {
+      cv.wait(lock);
+    }
+    if (status.ok()) {
+      return reply.message();
+    } else {
+      std::cout << status.error_code() << ": " << status.error_message()
+                << "\n";
+      return "RPC failed";
+    }
+  }
+
+ private:
+  std::unique_ptr<Greeter::Stub> stub_;
+};
+
+int main(int argc, char** argv) {
+  absl::ParseCommandLine(argc, argv);
+  std::string target_str = absl::GetFlag(FLAGS_target);
+  GreeterClient greeter(
+      grpc::CreateChannel(target_str, grpc::InsecureChannelCredentials()));
+  std::string user("world");
+  // First send an RPC without wait_for_ready. If the server is not running,
+  // this RPC will fail immediately.
+  std::cout << "Greeter received: "
+            << greeter.SayHello(user, /*wait_for_ready=*/false) << "\n";
+  std::cout << "\nWe will now send RPCs with wait_for_ready set. If the "
+               "server is not running already, please start it now.\n";
+  // Now send RPC with wait_for_ready for set. Even if the server is not
+  // running, the RPC will still wait for the deadline to expire before
+  // failing.
+  std::cout << "Greeter received: "
+            << greeter.SayHello(user, /*wait_for_ready=*/true) << "\n";
+
+  return 0;
+}