Is it possible to get metadata from arbitrary items in an Incremental Generator?

[Echo-8-ERA]

Let's say I have this in my project file:

  ...

  <PropertyGroup>
    <EmitCompilerGeneratedFiles>True</EmitCompilerGeneratedFiles>
  </PropertyGroup>

  <ItemGroup>
    <GlobalConst Include="MyConst1" Value="1"/>
    <GlobalConst Include="MyConst2" Value="2"/>
    <GlobalConst Include="MyConst3" Value="3"/>
  </ItemGroup>

  <ItemGroup>
    <CompilerVisibleItemMetadata Include="GlobalConst" MetadataName="Value"/>
  </ItemGroup>

  ...

How would I go about reading that information in an incremental generator?

I've tried adapting the example in the incremental generators cookbook but nothing gets output. If I replace GlobalConst with AdditionalFiles, it works.

Imports System.Text
Imports System.Threading
Imports Microsoft.CodeAnalysis
Imports Microsoft.CodeAnalysis.Diagnostics
Imports System.Collections.Immutable

<Generator(LanguageNames.VisualBasic)>
Public Class GlobalConstantGenerator
	Implements IIncrementalGenerator

	Public Sub Initialize(ByVal context As IncrementalGeneratorInitializationContext) Implements IIncrementalGenerator.Initialize

		Dim constants = context.AdditionalTextsProvider.Combine(context.AnalyzerConfigOptionsProvider).SelectMany(
				Function(pair, ct) 
					Dim value As String
					pair.Right.GetOptions(pair.Left).TryGetValue("build_metadata.GlobalConst.Value", value)
					Return { (pair.Left.Path, value) }
				End Function).Collect()

		context.RegisterSourceOutput(constants,
				Sub(ctx, data)
						Dim src = New StringBuilder()
						For Each kvp In data
							src.Append($"' ""{kvp.Item1}"" = ""{kvp.Item2}""{vbCrLf}")
						Next
						ctx.AddSource("Dump.GlobalConst.g.vb", src.ToString())
				End Sub)

	End Sub

End Class