fix: add default for metrics to fix linting error

Author: tyrchen

src/metrics.rs

@@ -12,6 +12,12 @@ pub struct Metrics {
     data: Arc<Mutex<HashMap<String, i64>>>,
 }
 
+impl Default for Metrics {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl Metrics {
     pub fn new() -> Self {
         Metrics {