examples: add an example to illustrate the usage of stats handler

examples/features/stats_monitoring/README.md

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+# Stats Monitoring Handler
+
+This example demonstrates the use of the [`stats`](https://pkg.go.dev/google.golang.org/grpc/stats) package for reporting various  
+network and RPC stats.  
+_Note that all fields are READ-ONLY and the APIs of the `stats` package are  
+experimental_.
+
+## Try it
+
+```
+go run server/main.go
+```
+
+```
+go run client/main.go
+```
+
+## Explanation
+
+gRPC provides a mechanism to hook on to various events (phases) of the  
+request-response network cycle through the [`stats.Handler`](https://pkg.go.dev/google.golang.org/grpc/stats#Handler) interface. To access  
+these events, a concrete type that implements `stats.Handler` should be passed to  
+`grpc.WithStatsHandler()` on the client side and `grpc.StatsHandler()` on the  
+server side.
+
+The `HandleRPC(context.Context, RPCStats)` method on `stats.Handler` is called  
+multiple times during a request-response cycle, and various event stats are  
+passed to its `RPCStats` parameter (an interface). The concrete types that  
+implement this interface are: `*stats.Begin`, `*stats.InHeader`, `*stats.InPayload`,  
+`*stats.InTrailer`, `*stats.OutHeader`, `*stats.OutPayload`, `*stats.OutTrailer`, and  
+`*stats.End`. The order of these events differs on client and server.
+
+Similarly, the `HandleConn(context.Context, ConnStats)` method on `stats.Handler`  
+is called twice, once at the beginning of the connection with `*stats.ConnBegin`  
+and once at the end with `*stats.ConnEnd`.
+
+The [`stats.Handler`](https://pkg.go.dev/google.golang.org/grpc/stats#Handler) interface also provides  
+`TagRPC(context.Context, *RPCTagInfo) context.Context` and  
+`TagConn(context.Context, *ConnTagInfo) context.Context` methods. These methods  
+are mainly used to attach network related information to the given context.
+
+The `TagRPC(context.Context, *RPCTagInfo) context.Context` method returns a  
+context from which the context used for the rest lifetime of the RPC will be  
+derived. This behavior is consistent between the gRPC client and server.
+
+The context returned from  
+`TagConn(context.Context, *ConnTagInfo) context.Context` has varied lifespan:
+
+- In the gRPC client:  
+  The context used for the rest lifetime of the RPC will NOT be derived from  
+  this context. Hence the information attached to this context can only be  
+  consumed by `HandleConn(context.Context, ConnStats)` method.
+- In the gRPC server:  
+  The context used for the rest lifetime of the RPC will be derived from  
+  this context.
+
+NOTE: The [stats](https://pkg.go.dev/google.golang.org/grpc/stats) package should only be used for network monitoring purposes,  
+and not as an alternative to [interceptors](https://github.com/grpc/grpc-go/blob/master/examples/features/metadata).

examples/features/stats_monitoring/client/main.go

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+/*
+ *
+ * Copyright 2022 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.
+ *
+ */
+
+// Binary client is an example client to illustrate the use of the stats handler.
+package main
+
+import (
+	"context"
+	"flag"
+	"log"
+	"time"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+
+	echogrpc "google.golang.org/grpc/examples/features/proto/echo"
+	echopb "google.golang.org/grpc/examples/features/proto/echo"
+	"google.golang.org/grpc/examples/features/stats_monitoring/statshandler"
+)
+
+var addr = flag.String("addr", "localhost:50051", "the address to connect to")
+
+func main() {
+	flag.Parse()
+	opts := []grpc.DialOption{
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+		grpc.WithStatsHandler(statshandler.New()),
+	}
+	conn, err := grpc.Dial(*addr, opts...)
+	if err != nil {
+		log.Fatalf("failed to connect to server %q: %v", *addr, err)
+	}
+	defer conn.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	c := echogrpc.NewEchoClient(conn)
+
+	resp, err := c.UnaryEcho(ctx, &echopb.EchoRequest{Message: "stats handler demo"})
+	if err != nil {
+		log.Fatalf("unexpected error from UnaryEcho: %v", err)
+	}
+	log.Printf("RPC response: %s", resp.Message)
+}

examples/features/stats_monitoring/server/main.go

@@ -0,0 +1,62 @@
+/*
+ *
+ * Copyright 2022 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.
+ *
+ */
+
+// Binary server is an example server to illustrate the use of the stats handler.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"net"
+	"time"
+
+	"google.golang.org/grpc"
+
+	echogrpc "google.golang.org/grpc/examples/features/proto/echo"
+	echopb "google.golang.org/grpc/examples/features/proto/echo"
+	"google.golang.org/grpc/examples/features/stats_monitoring/statshandler"
+)
+
+var port = flag.Int("port", 50051, "the port to serve on")
+
+type server struct {
+	echogrpc.UnimplementedEchoServer
+}
+
+func (s *server) UnaryEcho(ctx context.Context, req *echopb.EchoRequest) (*echopb.EchoResponse, error) {
+	time.Sleep(2 * time.Second)
+	return &echopb.EchoResponse{Message: req.Message}, nil
+}
+
+func main() {
+	flag.Parse()
+	lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))
+	if err != nil {
+		log.Fatalf("failed to listen on port %d: %v", *port, err)
+	}
+	log.Printf("server listening at %v\n", lis.Addr())
+
+	s := grpc.NewServer(grpc.StatsHandler(statshandler.New()))
+	echogrpc.RegisterEchoServer(s, &server{})
+
+	if err := s.Serve(lis); err != nil {
+		log.Fatalf("failed to serve: %v", err)
+	}
+}

examples/features/stats_monitoring/statshandler/handler.go

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+/*
+ *
+ * Copyright 2022 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 statshandler is an example pkg to illustrate the use of the stats handler.
+package statshandler
+
+import (
+	"context"
+	"log"
+	"net"
+	"path/filepath"
+
+	"google.golang.org/grpc/stats"
+)
+
+// Handler implements [stats.Handler](https://pkg.go.dev/google.golang.org/grpc/stats#Handler) interface.
+type Handler struct{}
+
+type connStatCtxKey struct{}
+
+// TagConn can attach some information to the given context.
+// The context used in HandleConn for this connection will be derived from the context returned.
+// In the gRPC client:
+// The context used in HandleRPC for RPCs on this connection will be the user's context and NOT derived from the context returned here.
+// In the gRPC server:
+// The context used in HandleRPC for RPCs on this connection will be derived from the context returned here.
+func (st *Handler) TagConn(ctx context.Context, stat *stats.ConnTagInfo) context.Context {
+	log.Printf("[TagConn] [%T]: %+[1]v", stat)
+	return context.WithValue(ctx, connStatCtxKey{}, stat)
+}
+
+// HandleConn processes the Conn stats.
+func (st *Handler) HandleConn(ctx context.Context, stat stats.ConnStats) {
+	var rAddr net.Addr
+	if s, ok := ctx.Value(connStatCtxKey{}).(*stats.ConnTagInfo); ok {
+		rAddr = s.RemoteAddr
+	}
+
+	if stat.IsClient() {
+		log.Printf("[server addr: %s] [HandleConn] [%T]: %+[2]v", rAddr, stat)
+	} else {
+		log.Printf("[client addr: %s] [HandleConn] [%T]: %+[2]v", rAddr, stat)
+	}
+}
+
+type rpcStatCtxKey struct{}
+
+// TagRPC can attach some information to the given context.
+// The context used for the rest lifetime of the RPC will be derived from the returned context.
+func (st *Handler) TagRPC(ctx context.Context, stat *stats.RPCTagInfo) context.Context {
+	log.Printf("[TagRPC] [%T]: %+[1]v", stat)
+	return context.WithValue(ctx, rpcStatCtxKey{}, stat)
+}
+
+// HandleRPC processes the RPC stats. Note: All stat fields are read-only.
+func (st *Handler) HandleRPC(ctx context.Context, stat stats.RPCStats) {
+	var sMethod string
+	if s, ok := ctx.Value(rpcStatCtxKey{}).(*stats.RPCTagInfo); ok {
+		sMethod = filepath.Base(s.FullMethodName)
+	}
+
+	var cAddr net.Addr
+	// for gRPC clients, key connStatCtxKey{} will not be present in HandleRPC's context.
+	if s, ok := ctx.Value(connStatCtxKey{}).(*stats.ConnTagInfo); ok {
+		cAddr = s.RemoteAddr
+	}
+
+	if stat.IsClient() {
+		log.Printf("[server method: %s] [HandleRPC] [%T]: %+[2]v", sMethod, stat)
+	} else {
+		log.Printf("[client addr: %s] [HandleRPC] [%T]: %+[2]v", cAddr, stat)
+	}
+}
+
+// New returns a new implementation of [stats.Handler](https://pkg.go.dev/google.golang.org/grpc/stats#Handler) interface.
+func New() *Handler {
+	return &Handler{}
+}