Add etcd to default registries when plugin is loaded (#2150)

Co-authored-by: Andrew Jones <andrew@gotoblink.com>

plugins/registry/etcd/etcd.go

@@ -13,6 +13,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/asim/go-micro/v3/cmd"
 	"github.com/asim/go-micro/v3/logger"
 	"github.com/asim/go-micro/v3/registry"
 	hash "github.com/mitchellh/hashstructure"
@@ -34,6 +35,10 @@ type etcdRegistry struct {
 	leases   map[string]clientv3.LeaseID
 }
 
+func init() {
+	cmd.DefaultRegistries["etcd"] = NewRegistry
+}
+
 func NewRegistry(opts ...registry.Option) registry.Registry {
 	e := &etcdRegistry{
 		options:  registry.Options{},