registry/nacos: reduce twice string copies (#1681)

* registry/nacos: reduce two string copies

* registry/nacos: update example

contrib/registry/nacos/README.md

@@ -1,27 +1,29 @@
 # Nacos Registry
 
+## example
+### server
 ```go
+package main
+
 import (
-		"github.com/go-kratos/kratos/v2"
+	"log"
-		"github.com/go-kratos/kratos/v2/transport/grpc"
 
 	"github.com/nacos-group/nacos-sdk-go/clients"
 	"github.com/nacos-group/nacos-sdk-go/common/constant"
 	"github.com/nacos-group/nacos-sdk-go/vo"
+
+	"github.com/go-kratos/kratos/contrib/registry/nacos/v2"
+	"github.com/go-kratos/kratos/v2"
 )
 
+func main() {
 	sc := []constant.ServerConfig{
 		*constant.NewServerConfig("127.0.0.1", 8848),
 	}
 	
-cc := constant.ClientConfig{
-	NamespaceId:         "public",
-	TimeoutMs:           5000,
-}
 
 	client, err := clients.NewNamingClient(
 		vo.NacosClientParam{
-		ClientConfig:  &cc,
 			ServerConfigs: sc,
 		},
 	)
@@ -40,6 +42,42 @@ app := kratos.New(
 	if err := app.Run(); err != nil {
 		log.Fatal(err)
 	}
+}
+```
+### client
+```go
+package main
+
+import (
+	"context"
+	"log"
+
+	"github.com/nacos-group/nacos-sdk-go/clients"
+	"github.com/nacos-group/nacos-sdk-go/common/constant"
+	"github.com/nacos-group/nacos-sdk-go/vo"
+
+	"github.com/go-kratos/kratos/contrib/registry/nacos/v2"
+	"github.com/go-kratos/kratos/v2/transport/grpc"
+)
+
+func main() {
+
+	cc := constant.ClientConfig{
+		NamespaceId: "public",
+		TimeoutMs:   5000,
+	}
+
+	client, err := clients.NewNamingClient(
+		vo.NacosClientParam{
+			ClientConfig: &cc,
+		},
+	)
+
+	if err != nil {
+		log.Panic(err)
+	}
+
+	r := nacos.New(client)
 
 	// client
 	conn, err := grpc.DialInsecure(
@@ -47,4 +85,6 @@ conn, err := grpc.DialInsecure(
 		grpc.WithEndpoint("discovery:///helloworld"),
 		grpc.WithDiscovery(r),
 	)
+	defer conn.Close()
+}
 ```

contrib/registry/nacos/registry.go

@@ -79,7 +79,7 @@ func New(cli naming_client.INamingClient, opts ...Option) (r *Registry) {
 }
 
 // Register the registration.
-func (r *Registry) Register(ctx context.Context, si *registry.ServiceInstance) error {
+func (r *Registry) Register(_ context.Context, si *registry.ServiceInstance) error {
 	if si.Name == "" {
 		return fmt.Errorf("kratos/nacos: serviceInstance.name cannot is empty")
 	}
@@ -96,15 +96,20 @@ func (r *Registry) Register(ctx context.Context, si *registry.ServiceInstance) e
 		if err != nil {
 			return err
 		}
+		var rmd map[string]string
 		if si.Metadata == nil {
-			si.Metadata = make(map[string]string, 2)
+			rmd = map[string]string{
+				"kind":    u.Scheme,
+				"version": si.Version,
 			}
-		si.Metadata["kind"] = u.Scheme
+		} else {
-		si.Metadata["version"] = si.Version
+			rmd = make(map[string]string, len(si.Metadata)+2)
-		rmd := make(map[string]string, len(si.Metadata))
 			for k, v := range si.Metadata {
 				rmd[k] = v
 			}
+			rmd["kind"] = u.Scheme
+			rmd["version"] = si.Version
+		}
 		_, e := r.cli.RegisterInstance(vo.RegisterInstanceParam{
 			Ip:          host,
 			Port:        uint64(p),
@@ -125,7 +130,7 @@ func (r *Registry) Register(ctx context.Context, si *registry.ServiceInstance) e
 }
 
 // Deregister the registration.
-func (r *Registry) Deregister(ctx context.Context, service *registry.ServiceInstance) error {
+func (r *Registry) Deregister(_ context.Context, service *registry.ServiceInstance) error {
 	for _, endpoint := range service.Endpoints {
 		u, err := url.Parse(endpoint)
 		if err != nil {
@@ -159,7 +164,7 @@ func (r *Registry) Watch(ctx context.Context, serviceName string) (registry.Watc
 }
 
 // GetService return the service instances in memory according to the service name.
-func (r *Registry) GetService(ctx context.Context, serviceName string) ([]*registry.ServiceInstance, error) {
+func (r *Registry) GetService(_ context.Context, serviceName string) ([]*registry.ServiceInstance, error) {
 	res, err := r.cli.SelectInstances(vo.SelectInstancesParam{
 		ServiceName: serviceName,
 		HealthyOnly: true,