Link to the code that reproduces this issue or a replay of the bug

https://github.com/wiwo-dev/generatemetadata-loading-issue
https://codesandbox.io/p/github/wiwo-dev/generatemetadata-loading-issue/main

To Reproduce

1. Run the application using the command: yarn run dev.

2. Navigate to the root path (/). Here, you'll find two lists of "users" retrieved from jsonplaceholder. The list at the top has the problem as it uses generateMetadata. The one at the bottom doesn't use generateMetadata.

3. The top list contains links to /user/[userId], and these links have a generateMetadata function that fetches data. Upon clicking these links, for the initial 3 seconds, there is no visible loading indication, making users believe that something might be wrong with the link.

4. In contrast, if you click on any of the links in the bottom list, which leads to /user-no-metadata/[userId] that doesn't use generateMetadata, you'll notice that a Loading state is triggered by Suspense, providing a clear loading indication.

Current vs. Expected behavior

Current behaviour
When a user navigates to a page containing a generateMetadata function that fetches data from a slowly working API, there's a problem. The application appears to hang, providing no indication that it's actively loading. Notably, the fetch operation within generateMetadata doesn't trigger React's Suspense, and the browser also fails to display any loading indicators.

To simulate a slow API response, a setTimeout is intentionally used within the getUsers function.

I encountered this issue while working on a project with a tens of thousands number of subpages, and it's highly likely that a given subpage will be opened for the first time and only once, with new entries continually being added.

Expected behaviour
It should trigger the Suspense or show any kind of loading indicator in the browser so the user knows that something is happening under the hood and wait for the page to be displayed.

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
      Platform: darwin
      Arch: arm64
      Version: Darwin Kernel Version 22.4.0: Mon Mar  6 21:00:41 PST 2023; root:xnu-8796.101.5~3/RELEASE_ARM64_T8103
    Binaries:
      Node: 18.7.0
      npm: 8.15.0
      Yarn: 1.22.15
      pnpm: 6.11.0
    Relevant Packages:
      next: 13.4.20-canary.36
      eslint-config-next: N/A
      react: 18.2.0
      react-dom: 18.2.0
      typescript: 5.1.3
    Next.js Config:
      output: N/A

Which area(s) are affected? (Select all that apply)

Metadata (metadata, generateMetadata, next/head)

Additional context

Is there a way to display a loading state or any form of indication that something is loading while still utilising the generateMetadata function to fetch metadata information from a slow API?