clippy all codes (#1214)

1. clippy code
2. add fmt and clippy check in ci

easytier/src/common/stats_manager.rs

@@ -277,6 +277,12 @@ pub struct UnsafeCounter {
     value: UnsafeCell<u64>,
 }
 
+impl Default for UnsafeCounter {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl UnsafeCounter {
     pub fn new() -> Self {
         Self {
@@ -396,14 +402,15 @@ impl MetricKey {
     fn new(name: MetricName, labels: LabelSet) -> Self {
         Self { name, labels }
     }
+}
 
-    /// Generate a string representation for this metric key
-    fn to_string(&self) -> String {
+impl fmt::Display for MetricKey {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let label_str = self.labels.to_key();
         if label_str.is_empty() {
-            self.name.to_string()
+            f.write_str(self.name.to_string().as_str())
         } else {
-            format!("{}[{}]", self.name, label_str)
+            f.write_str(format!("{}[{}]", self.name, label_str).as_str())
         }
     }
 }