Using PageLoader for multiple endpoints can cause the loss of endpoint metadata

### Describe the bug

I'm using `PageLoader` per @davidfowl's [recommendation](https://twitter.com/davidfowl/status/1394477907607654403) to get the full metadata for multiple endpoints on a single razor page. I've noticed that after I've loaded an endpoint via `PageLoader` that hasn't been visited via an HTTP request, then there is metadata missing that would normally be there without using `PageLoader`.

For example, if I call `endpoints.MapRazorPages().WithMetadata("Some Metadata")` and then inspect the endpoint in middleware during a normal request the custom metadata exists on the endpoint. But if I load the endpoint with `PageLoader` first before doing an actual HTTP request then the custom data does not exist on the endpoint.

During a normal request `PageLoaderMatcherPolicy` specifically calls `DefaultPageLoader.LoadAsync()` with the original endpoint metadata and then enriches it with the compiled metadata and then caches the value. Unfortunately, the `PageLoader.LoadAsync(PageActionDescriptor actionDescriptor, EndpointMetadataCollection endpointMetadata)` method is marked internal so I can't pass along the original endpoint metadata so the endpoint is cached with missing metadata. See my linked GitHub repo below for an example of this.

Is there a better way for me to get all the endpoints in my app with their full metadata other than calling `PageLoader`? Or should I be calling `PageLoader` differently?

### To Reproduce
See an example at https://github.com/jeffpapp/PageLoaderSample/tree/main/PageLoaderSample

If you run the sample project and follow the Go to Next Page links you will see that the third page is missing the custom metadata on it's endpoint because the second page called `PageLoader` which overrode the default behavior of `PageLoaderMatcherPolicy` and cached an endpoint without all the metadata you'd expect.

### Further technical details
- ASP.NET Core version: 5.0.6
- Include the output of `dotnet --info`
```
.NET SDK (reflecting any global.json):
Version: 5.0.100
Commit: 5044b93829

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19041
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.100\

Host (useful for support):
Version: 5.0.6
Commit: 478b2f8c0e

.NET SDKs installed:
3.1.402 [C:\Program Files\dotnet\sdk]
5.0.100 [C:\Program Files\dotnet\sdk]
5.0.203 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.15 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
https://aka.ms/dotnet-download
```
- The IDE (VS / VS Code/ VS4Mac) you're running on, and its version: VS 2019 16.9.6