docs: middleware metrics

README.md

@@ -393,6 +393,21 @@ export default async function Page() {
 }
 ```
 
+## Next.js middleware
+
+In `middleware.ts` you can use [`event.waitUntil()`](https://nextjs.org/docs/app/building-your-application/routing/middleware#waituntil-and-nextfetchevent) to reliably send metrics without delaying the response.
+
+```tsx
+export async function middleware(req: NextRequest, event: NextFetchEvent) {
+    // ...
+    const evaluated = evaluateFlags(definitions, context);
+    const flags = flagsClient(evaluated.toggles)
+    const isEnabled = flags.isEnabled("example-flag")
+
+    event.waitUntil(flags.sendMetrics())
+    // ...
+}
+```
 
 ## `setInterval` support (e.g. long-running Node process or AWS lambda)
 
@@ -436,5 +451,4 @@ npx @unleash/nextjs generate-types ./unleash.ts
 
 # Known limitation
 
-- In current interation **server-side SDK does not support metrics**.
 - When used server-side, this SDK does not support the "Hostname" and "IP" strategies. Use custom context fields and constraints instead.

example/src/middleware.ts

@@ -1,4 +1,4 @@
-import { NextRequest, NextResponse } from "next/server";
+import { type NextFetchEvent, type NextRequest, NextResponse } from "next/server";
 import { UNLEASH_API_PROXY_DEFINITIONS, UNLEASH_COOKIE_NAME } from "./utils";
 import { addBasePath } from "next/dist/client/add-base-path";
 import { flagsClient, evaluateFlags, randomSessionId } from "@unleash/nextjs";
@@ -7,7 +7,7 @@ export const config = {
     runtime: "experimental-edge",
 };
 
-export async function middleware(req: NextRequest) {
+export async function middleware(req: NextRequest, event: NextFetchEvent) {
     const sessionId =
         req.cookies.get(UNLEASH_COOKIE_NAME)?.value || randomSessionId();
     let res: NextResponse;
@@ -26,18 +26,20 @@ export async function middleware(req: NextRequest) {
 
         // Evaluate based on provided context
         const evaluated = evaluateFlags(definitions, context);
-        const variant = flagsClient(evaluated.toggles).getVariant("example-flag")
+        const flags = flagsClient(evaluated.toggles)
+        const variant = flags.getVariant("example-flag")
           ?.payload?.value;
 
         const newUrl = req.nextUrl.clone();
         // Redirect to variant
         newUrl.pathname = `/ab/${variant === "a" ? "a" : "b"}`;
         res = NextResponse.rewrite(newUrl);
+
+        event.waitUntil(flags.sendMetrics()) // Experimental, since 1.5.0-beta.3
     }
     else {
         res = NextResponse.next();
     }
     res.cookies.set(UNLEASH_COOKIE_NAME, sessionId);
     return res; 
 }
-