plugin/grpc: close clients on shutdown (#8582)

This PR closes from the plugin shutdown hook

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/grpc/proxy.go

@@ -37,6 +37,7 @@ type Proxy struct {
 	addr string
 
 	// connection
+	conn     *grpc.ClientConn
 	client   pb.DnsServiceClient
 	dialOpts []grpc.DialOption
 }
@@ -66,11 +67,19 @@ func newProxy(addr string, tlsConfig *tls.Config) (*Proxy, error) {
 	if err != nil {
 		return nil, err
 	}
+	p.conn = conn
 	p.client = pb.NewDnsServiceClient(conn)
 
 	return p, nil
 }
 
+func (p *Proxy) close() error {
+	if p.conn == nil {
+		return nil
+	}
+	return p.conn.Close()
+}
+
 // query sends the request and waits for a response.
 func (p *Proxy) query(ctx context.Context, req *dns.Msg) (*dns.Msg, error) {
 	start := time.Now()