docs: instrumentationClientInject

docs/01-app/03-api-reference/03-file-conventions/instrumentation-client.mdx

@@ -87,6 +87,10 @@ The `instrumentation-client.js` file executes at a specific point in the applica
 
 This timing makes it ideal for setting up error tracking, analytics, and performance monitoring that needs to capture early application lifecycle events.
 
+## See also
+
+`next.config.js` plugins (for example, wrappers like `withSentry`) can register their own client instrumentation module via the [`instrumentationClientInject`](/docs/app/api-reference/config/next-config-js/instrumentationClientInject) option. Injected modules run before this file, in array order, and may export their own `onRouterTransitionStart`. Application code should continue to use this file convention directly.
+
 ## Examples
 
 ### Error tracking

docs/01-app/03-api-reference/05-config/01-next-config-js/instrumentationClientInject.mdx

@@ -0,0 +1,68 @@
+---
+title: instrumentationClientInject
+description: Inject additional client-side instrumentation modules before the user's `instrumentation-client.{js,ts}` file.
+---
+
+`instrumentationClientInject` is a list of modules that are imported on the client for their side effects before the user's [`instrumentation-client.{js,ts}`](/docs/app/api-reference/file-conventions/instrumentation-client) file runs, ahead of React hydration.
+
+This option is primarily intended for **`next.config.js` plugins** (for example, wrappers like `withSentry` or `withAnalytics` that extend a project's config). It lets such a plugin inject its own client instrumentation module — including a navigation hook — without requiring every project to author or modify an `instrumentation-client` file. Application code should generally continue to use the [`instrumentation-client.{js,ts}`](/docs/app/api-reference/file-conventions/instrumentation-client) file convention directly.
+
+A plugin typically appends its own module to whatever the project already has configured:
+
+```js filename="withMyInstrumentation.js"
+module.exports = function withMyInstrumentation(nextConfig = {}) {
+  return {
+    ...nextConfig,
+    instrumentationClientInject: [
+      ...(nextConfig.instrumentationClientInject ?? []),
+      'my-instrumentation-package/client',
+    ],
+  }
+}
+```
+
+It can also be set directly:
+
+```js filename="next.config.js"
+/** @type {import('next').NextConfig} */
+module.exports = {
+  instrumentationClientInject: [
+    'my-analytics-package',
+    './lib/sentry-client.js',
+  ],
+}
+```
+
+Each entry is one of:
+
+- A bare npm package name, resolved from the project's `node_modules`.
+- A path relative to the project root.
+
+## Execution order
+
+Modules run on the client in this order:
+
+1. Each entry in `instrumentationClientInject`, in array order.
+2. The project's `instrumentation-client.{js,ts}` file, if present.
+3. React hydration.
+
+## Router navigation hook
+
+Each injected module may optionally export an `onRouterTransitionStart` function with the same signature as the one documented for the [`instrumentation-client` file convention](/docs/app/api-reference/file-conventions/instrumentation-client#router-navigation-tracking). Next.js composes a single hook that fans out to every exported `onRouterTransitionStart` on each navigation, calling them in array order, with the user file's hook running last.
+
+```js filename="lib/sentry-client.js"
+// Side-effectful setup runs at load time.
+setupSentry()
+
+export function onRouterTransitionStart(url, navigationType) {
+  recordNavigationBreadcrumb(url, navigationType)
+}
+```
+
+Modules that do not export `onRouterTransitionStart` are skipped during navigation.
+
+## Version history
+
+| Version   | Changes                                  |
+| --------- | ---------------------------------------- |
+| `v16.3.0` | `instrumentationClientInject` introduced |