initial working exn tracing

README.md

@@ -2,11 +2,13 @@
 
 Context aware errors.
 
+Trying to get exn into structured output for tracing_subscriber.
+
 https://github.com/fast/exn
 
 https://fast.github.io/blog/stop-forwarding-errors-start-designing-them/
 
-example output:
+example default output:
 
 ```
 the program died, at src/main.rs:35:34
@@ -15,3 +17,30 @@ the program died, at src/main.rs:35:34
 |
 |-> Permission denied (os error 13), at src/main.rs:79:43
 ```
+
+example structured output
+
+```json
+{
+  "fields": {
+    "error": {
+      "causes": [
+        {
+          "causes": [
+            {
+              "location": "src/main.rs:86:43",
+              "message": "Permission denied (os error 13)"
+            }
+          ],
+          "location": "src/main.rs:86:43",
+          "message": "failed to create file: res/foo.txt"
+        }
+      ],
+      "location": "src/main.rs:41:34",
+      "message": "the program died"
+    }
+  },
+  "level": "ERROR",
+  "target": "playground_exn",
+  "timestamp": "2026-03-22T01:19:11.256874822+00:00"
+}```