Make sure that default is respected when [custom_tracing] is missing (

#862)

custom_tracing/src/settings.rs

@@ -1,12 +1,20 @@
 use serde::{Deserialize, Serialize};
 
-#[derive(Debug, Serialize, Deserialize, Clone, Default)]
+#[derive(Debug, Serialize, Deserialize, Clone)]
 pub struct Settings {
     /// File name to be watched by custom tracing
     #[serde(default = "default_tracing_cfg_file")]
     pub tracing_cfg_file: String,
 }
 
+impl Default for Settings {
+    fn default() -> Self {
+        Self {
+            tracing_cfg_file: default_tracing_cfg_file(),
+        }
+    }
+}
+
 pub fn default_tracing_cfg_file() -> String {
     "tracing.cfg".to_string()
 }