fix(instrumentation): handle "/" route correctly in App Router (#17) (#18)

* fix(instrumentation): handle "/" route correctly in App Router (#17)

* chore: remove leftover issue reference comment (#17)

* refactor: use isAppRouter helper in route check

Author: zto-sbenning

packages/nextjs-cache-handler/src/instrumentation/register-initial-cache.ts

@@ -231,9 +231,13 @@ export async function registerInitialCache(
     router: Router,
     revalidate: Revalidate,
   ) {
-    const pathToRouteFiles = path.join(serverDistDir, router, cachePath);
-
     const isAppRouter = router === "app";
+    
+    if (isAppRouter && cachePath === "/") {
+      cachePath = "/index";
+    }
+
+    const pathToRouteFiles = path.join(serverDistDir, router, cachePath);
 
     let lastModified: number | undefined;