log: log dumped player number

sdgb-cli/src/main.rs

@@ -223,6 +223,7 @@ async fn main() -> Result<(), Box<dyn snafu::Error>> {
             file.lock()?;
             let writer = BufWriter::new(file);
             serde_json::to_writer(writer, &user_ids)?;
+            info!("dumped {} user id", user_ids.len());
         }
 
         commands::Commands::Userdata { user_id } => {