examples: add example to show how to use the health service (#3381)

examples/features/health/README.md

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+# Health
+
+gRPC provides a health library to communicate a system's health to their clients.
+It works by providing a service definition via the [health/v1](https://github.com/grpc/grpc-proto/blob/master/grpc/health/v1/health.proto) api.
+
+By using the health library, clients can gracefully avoid using servers as they encounter issues. 
+Most languages provide an implementation out of box, making it interoperable between systems.
+
+## Try it
+
+```
+go run server/main.go -port=50051 -sleep=5s
+go run server/main.go -port=50052 -sleep=10s
+```
+
+```
+go run client/main.go
+```
+
+## Explanation
+
+### Client
+
+Clients have two ways to monitor a servers health.
+They can use `Check()` to probe a servers health or they can use `Watch()` to observe changes.
+
+In most cases, clients do not need to directly check backend servers.
+Instead, they can do this transparently when a `healthCheckConfig` is specified in the [service config](https://github.com/grpc/proposal/blob/master/A17-client-side-health-checking.md#service-config-changes).
+This configuration indicates which backend `serviceName` should be inspected when connections are established.
+An empty string (`""`) typically indicates the overall health of a server should be reported.
+
+```go
+// import grpc/health to enable transparent client side checking 
+import _ "google.golang.org/grpc/health"
+
+// set up appropriate service config
+serviceConfig := grpc.WithDefaultServiceConfig(`{
+  "loadBalancingPolicy": "round_robin",
+  "healthCheckConfig": {
+    "serviceName": ""
+  }
+}`)
+
+conn, err := grpc.Dial(..., serviceConfig)
+```
+
+See [A17 - Client-Side Health Checking](https://github.com/grpc/proposal/blob/master/A17-client-side-health-checking.md) for more details.
+
+### Server
+
+Servers control their serving status.
+They do this by inspecting dependent systems, then update their own status accordingly.
+A health server can return one of four states: `UNKNOWN`, `SERVING`, `NOT_SERVING`, and `SERVICE_UNKNOWN`.
+
+`UNKNOWN` indicates the current state is not yet known.
+This state is often seen at the start up of a server instance.
+
+`SERVING` means that the system is healthy and ready to service requests.
+Conversely, `NOT_SERVING` indicates the system is unable to service requests at the time.
+
+`SERVICE_UNKNOWN` communicates the `serviceName` requested by the client is not known by the server.
+This status is only reported by the `Watch()` call. 
+
+A server may toggle its health using `healthServer.SetServingStatus("serviceName", servingStatus)`.

examples/features/health/client/main.go

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+/*
+ *
+ * Copyright 2020 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.
+ *
+ */
+
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"time"
+
+	"google.golang.org/grpc"
+	pb "google.golang.org/grpc/examples/features/proto/echo"
+	_ "google.golang.org/grpc/health"
+	"google.golang.org/grpc/resolver"
+	"google.golang.org/grpc/resolver/manual"
+)
+
+var serviceConfig = `{
+	"loadBalancingPolicy": "round_robin",
+	"healthCheckConfig": {
+		"serviceName": ""
+	}
+}`
+
+func callUnaryEcho(c pb.EchoClient) {
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+	r, err := c.UnaryEcho(ctx, &pb.EchoRequest{})
+	if err != nil {
+		fmt.Println("UnaryEcho: _, ", err)
+	} else {
+		fmt.Println("UnaryEcho: ", r.GetMessage())
+	}
+}
+
+func main() {
+	flag.Parse()
+
+	r, cleanup := manual.GenerateAndRegisterManualResolver()
+	defer cleanup()
+	r.InitialState(resolver.State{
+		Addresses: []resolver.Address{
+			{Addr: "localhost:50051"},
+			{Addr: "localhost:50052"},
+		},
+	})
+
+	address := fmt.Sprintf("%s:///unused", r.Scheme())
+
+	options := []grpc.DialOption{
+		grpc.WithInsecure(),
+		grpc.WithBlock(),
+		grpc.WithDefaultServiceConfig(serviceConfig),
+	}
+
+	conn, err := grpc.Dial(address, options...)
+	if err != nil {
+		log.Fatalf("did not connect %v", err)
+	}
+	defer conn.Close()
+
+	echoClient := pb.NewEchoClient(conn)
+
+	for {
+		callUnaryEcho(echoClient)
+		time.Sleep(time.Second)
+	}
+}

examples/features/health/server/main.go

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+/*
+ *
+ * Copyright 2020 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.
+ *
+ */
+
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"net"
+	"time"
+
+	"google.golang.org/grpc"
+	pb "google.golang.org/grpc/examples/features/proto/echo"
+	"google.golang.org/grpc/health"
+	healthpb "google.golang.org/grpc/health/grpc_health_v1"
+)
+
+var (
+	port  = flag.Int("port", 50051, "the port to serve on")
+	sleep = flag.Duration("sleep", time.Second*5, "duration between changes in health")
+
+	system = "" // empty string represents the health of the system
+)
+
+type echoServer struct {
+	pb.UnimplementedEchoServer
+}
+
+func (e *echoServer) UnaryEcho(ctx context.Context, req *pb.EchoRequest) (*pb.EchoResponse, error) {
+	return &pb.EchoResponse{
+		Message: fmt.Sprintf("hello from localhost:%d", *port),
+	}, nil
+}
+
+var _ pb.EchoServer = &echoServer{}
+
+func main() {
+	flag.Parse()
+
+	lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))
+	if err != nil {
+		log.Fatalf("failed to listen: %v", err)
+	}
+
+	s := grpc.NewServer()
+	healthcheck := health.NewServer()
+	healthpb.RegisterHealthServer(s, healthcheck)
+	pb.RegisterEchoServer(s, &echoServer{})
+
+	go func() {
+		// asynchronously inspect dependencies and toggle serving status as needed
+		next := healthpb.HealthCheckResponse_SERVING
+
+		for {
+			healthcheck.SetServingStatus(system, next)
+
+			if next == healthpb.HealthCheckResponse_SERVING {
+				next = healthpb.HealthCheckResponse_NOT_SERVING
+			} else {
+				next = healthpb.HealthCheckResponse_SERVING
+			}
+
+			time.Sleep(*sleep)
+		}
+	}()
+
+	if err := s.Serve(lis); err != nil {
+		log.Fatalf("failed to serve: %v", err)
+	}
+}