`useReportWebVitals` doesn't report Next.js custom metrics (such as `Next.js-hydration`)

### Link to the code that reproduces this issue https://github.com/keyz/nextjs-web-vitals-repro ### To Reproduce 1. Clone minimal repro (https://github.com/keyz/nextjs-web-vitals-repro/) 2. Run `npm run dev` 3. Observe in console that 1. The `useReportWebVitals(...)` hook does NOT report custom metrics such as `Next.js-hydration` 2. The `reportWebVitals(...)` export does, with a different type/shape of output | `useReportWebVitals` hook | `reportWebVitals` export | |---------------------------|--------------------------| | <img width="592" alt="useReportWebVitals" src="https://github.com/vercel/next.js/assets/2268452/b8b7ddde-b948-41e8-b95a-ef0abc475c68"> | <img width="574" alt="reportWebVitals" src="https://github.com/vercel/next.js/assets/2268452/6dafd5f9-2d50-4c3a-bb25-56022ae6ed8a"> | Here's the code: https://github.com/keyz/nextjs-web-vitals-repro/blob/7fe8f2bdf72224d836d1dc1de3cd5105a0eba2ef/src/pages/_app.tsx#L4-L14 ```tsx // _app.tsx import type { AppProps } from "next/app"; import { useReportWebVitals } from "next/web-vitals"; export default function App({ Component, pageProps }: AppProps) { useReportWebVitals((metric) => { console.log("1/ From `useReportWebVitals` hook:", metric); }); return <Component {...pageProps} />; } export function reportWebVitals(metric: any) { console.log("2/ From `reportWebVitals` export:", metric); } ``` ### Current vs. Expected behavior 1. The docs claim that the `useReportWebVitals` hook reports Next.js custom metrics, but it actually doesn't: https://nextjs.org/docs/pages/api-reference/functions/use-report-web-vitals#custom-metrics 2. There is no mention of `export reportWebVitals() {}` in the docs and whether it's supported or not ### Provide environment information ```bash $ npx next info Operating System: Platform: darwin Arch: x64 Version: Darwin Kernel Version 23.4.0: Wed Feb 21 21:44:54 PST 2024; root:xnu-10063.101.15~2/RELEASE_ARM64_T6031 Binaries: Node: 20.9.0 npm: 10.1.0 Yarn: 1.22.21 …