fix(diagnostics): fire asyncStart synchronously, asyncEnd in finally

maybeTraceMixed was firing asyncStart and asyncEnd back-to-back inside
the .then callback. That left no meaningful window between the two
events: for APM subscribers, 'async work begins' and 'async work
completes' fired in the same microtask, making the pair useless for
measuring the async duration.

Correct shape: asyncStart publishes synchronously as soon as we know
the operation is in-flight asynchronously (right after end), and
asyncEnd publishes in a .finally after settlement. Both the .then and
.finally are attached inside start.runStores, so AsyncLocalStorage
context is preserved end-to-end via async_hooks.

Event order now:
  sync path:   [start, end]
  async path:  [start, end, asyncStart, ..., asyncEnd]
  async error: [start, end, asyncStart, error, asyncEnd]

Mirrors the same fix in the FakeTracingChannel helper.

Author: logaretm

src/__testUtils__/fakeDiagnosticsChannel.ts

@@ -122,27 +122,22 @@ export class FakeTracingChannel implements MinimalTracingChannel {
         throw err;
       }
       this.end.publish(ctx);
-      return promise.then(
-        (result) => {
-          (ctx as { result: unknown }).result = result;
-          this.asyncStart.publish(ctx);
-          try {
+      this.asyncStart.publish(ctx);
+      return promise
+        .then(
+          (result) => {
+            (ctx as { result: unknown }).result = result;
             return result;
-          } finally {
-            this.asyncEnd.publish(ctx);
-          }
-        },
-        (err: unknown) => {
-          (ctx as { error: unknown }).error = err;
-          this.error.publish(ctx);
-          this.asyncStart.publish(ctx);
-          try {
+          },
+          (err: unknown) => {
+            (ctx as { error: unknown }).error = err;
+            this.error.publish(ctx);
             throw err;
-          } finally {
-            this.asyncEnd.publish(ctx);
-          }
-        },
-      );
+          },
+        )
+        .finally(() => {
+          this.asyncEnd.publish(ctx);
+        });
     });
   }
 }

src/diagnostics.ts

@@ -235,26 +235,22 @@ export function maybeTraceMixed<T>(
     }
 
     channel.end.publish(ctx);
-    return result.then(
-      (value) => {
-        ctx.result = value;
-        channel.asyncStart.publish(ctx);
-        try {
+    channel.asyncStart.publish(ctx);
+
+    return result
+      .then(
+        (value) => {
+          ctx.result = value;
           return value;
-        } finally {
-          channel.asyncEnd.publish(ctx);
-        }
-      },
-      (err: unknown) => {
-        ctx.error = err;
-        channel.error.publish(ctx);
-        channel.asyncStart.publish(ctx);
-        try {
+        },
+        (err: unknown) => {
+          ctx.error = err;
+          channel.error.publish(ctx);
           throw err;
-        } finally {
-          channel.asyncEnd.publish(ctx);
-        }
-      },
-    );
+        },
+      )
+      .finally(() => {
+        channel.asyncEnd.publish(ctx);
+      });
   });
 }