add stats metrics (#1207)

support new cli command `easytier-cli stats`

It's useful to find out which components are consuming bandwidth.

easytier/src/common/mod.rs

@@ -22,6 +22,7 @@ pub mod ifcfg;
 pub mod netns;
 pub mod network;
 pub mod scoped_task;
+pub mod stats_manager;
 pub mod stun;
 pub mod stun_codec_ext;
 pub mod token_bucket;