Use .micro domain for mdns

mdns.go

@@ -2,6 +2,8 @@
 package mdns
 
 import (
+	"context"
+
 	"github.com/micro/go-micro/registry"
 )
 
@@ -9,3 +11,13 @@ import (
 func NewRegistry(opts ...registry.Option) registry.Registry {
 	return registry.NewRegistry(opts...)
 }
+
+// Domain sets the mdnsDomain
+func Domain(d string) registry.Option {
+	return func(o *registry.Options) {
+		if o.Context == nil {
+			o.Context = context.Background()
+		}
+		o.Context = context.WithValue(o.Context, "mdns.domain", d)
+	}
+}