Add support for @(AssemblyMetadata) items that turn into assembly attributes

Given that `AssemblyMetadataAttribute` is such a common assembly-level attribute, this adds support
for specifying it directly via simple items, such as:

```
    <AssemblyMetadata Include="Foo" Value="Bar" />
    <AssemblyMetadata Include="Bar" Value="Baz" />
```

This also avoids having to learn the `_Parameter1` and `_Parameter2` syntax in `AssemblyAttribute` elements,
and is more similar to the way other higher-level properties like `AssemblyTitle` or `Product` are also
turned into assembly attributes.

Partially fixes #3166

The feature can be disabled by setting `$(GenerateAssemblyMetadataAttributes)` to `false`.

Author: kzu

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.GenerateAssemblyInfo.targets

@@ -33,6 +33,7 @@ Copyright (c) .NET Foundation. All rights reserved.
     <GenerateAssemblyTitleAttribute Condition="'$(GenerateAssemblyTitleAttribute)' == ''">true</GenerateAssemblyTitleAttribute>
     <GenerateAssemblyVersionAttribute Condition="'$(GenerateAssemblyVersionAttribute)' == ''">true</GenerateAssemblyVersionAttribute>
     <GenerateNeutralResourcesLanguageAttribute Condition="'$(GenerateNeutralResourcesLanguageAttribute)' == ''">true</GenerateNeutralResourcesLanguageAttribute>
+    <GenerateAssemblyMetadataAttributes Condition="'$(GenerateAssemblyMetadataAttributes)' == ''">true</GenerateAssemblyMetadataAttributes>
     <IncludeSourceRevisionInInformationalVersion Condition="'$(IncludeSourceRevisionInInformationalVersion)' == ''">true</IncludeSourceRevisionInInformationalVersion>
   </PropertyGroup>
 
@@ -94,6 +95,10 @@ Copyright (c) .NET Foundation. All rights reserved.
       <AssemblyAttribute Include="System.Resources.NeutralResourcesLanguageAttribute" Condition="'$(NeutralLanguage)' != '' and '$(GenerateNeutralResourcesLanguageAttribute)' == 'true'">
         <_Parameter1>$(NeutralLanguage)</_Parameter1>
       </AssemblyAttribute>
+      <AssemblyAttribute Include="System.Reflection.AssemblyMetadata" Condition="%(AssemblyMetadata.Identity) != '' and '$(GenerateAssemblyMetadataAttributes)' == 'true'">
+        <_Parameter1>%(AssemblyMetadata.Identity)</_Parameter1>
+        <_Parameter2>%(AssemblyMetadata.Value)</_Parameter2>
+      </AssemblyAttribute>      
     </ItemGroup>
   </Target>
 

src/Tests/Microsoft.NET.Build.Tests/GivenThatWeWantToControlGeneratedAssemblyInfo.cs

@@ -362,5 +362,64 @@ BuildCommand BuildProject(string buildNumber)
                 return command;
             }
         }
+
+        [Fact]
+        public void It_includes_assembly_metadata()
+        {
+            var testAsset = _testAssetsManager
+                .CopyTestAsset("HelloWorld")
+                .WithSource()
+                .WithTargetFramework("netstandard2.0")
+                .WithProjectChanges((path, project) =>
+                {
+                    var ns = project.Root.Name.Namespace;
+
+                    project.Root.Add(
+                        new XElement(ns + "ItemGroup",
+                            new XElement(ns + "AssemblyMetadata",
+                                new XAttribute("Include", "MetadataKey"),
+                                new XAttribute("Value", "MetadataValue"))));
+                });
+
+            new RestoreCommand(Log, testAsset.TestRoot).Execute().Should().Pass();
+
+            var buildCommand = new BuildCommand(Log, testAsset.TestRoot);
+            buildCommand.Execute().Should().Pass();
+
+            var assemblyPath = Path.Combine(buildCommand.GetOutputDirectory("netstandard2.0").FullName, "HelloWorld.dll");
+            var info = AssemblyInfo.Get(assemblyPath);
+
+            AssemblyInfo.Get(assemblyPath)["AssemblyMetadataAttribute"].Should().Be("MetadataKey:MetadataValue");
+        }
+
+        [Fact]
+        public void It_respects_out_out_of_assembly_metadata()
+        {
+            var testAsset = _testAssetsManager
+                .CopyTestAsset("HelloWorld")
+                .WithSource()
+                .WithTargetFramework("netstandard2.0")
+                .WithProjectChanges((path, project) =>
+                {
+                    var ns = project.Root.Name.Namespace;
+
+                    project.Root.Add(
+                        new XElement(ns + "PropertyGroup",
+                            new XElement(ns + "GenerateAssemblyMetadataAttributes", "false")),
+                        new XElement(ns + "ItemGroup",
+                            new XElement(ns + "AssemblyMetadata",
+                                new XAttribute("Include", "MetadataKey"),
+                                new XAttribute("Value", "MetadataValue"))));
+                });
+
+            new RestoreCommand(Log, testAsset.TestRoot).Execute().Should().Pass();
+
+            var buildCommand = new BuildCommand(Log, testAsset.TestRoot);
+            buildCommand.Execute().Should().Pass();
+
+            var assemblyPath = Path.Combine(buildCommand.GetOutputDirectory("netstandard2.0").FullName, "HelloWorld.dll");
+
+            Assert.False(AssemblyInfo.Get(assemblyPath).ContainsKey("AssemblyMetadataAttribute"));
+        }
     }
 }