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/setup.go

@@ -28,6 +28,7 @@ func setup(c *caddy.Controller) error {
 		g.Next = next // Set the Next field, so the plugin chaining works.
 		return g
 	})
+	c.OnShutdown(g.OnShutdown)
 
 	return nil
 }