Move metric refer to ASP.NET repo

[Rick-Anderson]

Moves the built-in metrics ref page from the dotnet/docs repo to the dotnet/AspNetCore.Docs repo.

Contributes to #35109 and dotnet/docs#45580
Prepares for dotnet/docs#45379

Copy/paste of https://github.com/dotnet/docs/blob/main/docs/core/diagnostics/built-in-metrics-aspnetcore.md and:

• Deletes all the available in .NET 8 disclaimers as this is now handled by monikers.
• changes meta-data

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Internal previews

📄 File	🔗 Preview link
aspnetcore/log-mon/metrics/built-in.md	aspnetcore/log-mon/metrics/built-in
aspnetcore/toc.yml	aspnetcore/toc

[Rick-Anderson]

@gewarren can I merge this? It's a copy/paste with only a few changes. I need to add the new .NET 10 metrics in the next PR.

[gewarren]

@gewarren can I merge this? It's a copy/paste with only a few changes. I need to add the new .NET 10 metrics in the next PR.

It's fine with me, but are @noahfalk and/or @JamesNK okay with moving them here?

[Rick-Anderson]

cc @MackinnonBuck

[noahfalk]

It's fine with me, but are @noahfalk and/or @JamesNK okay with moving them here?

I'm fine with it but can't speak for James.