Add netstandard2.0 compatibility to Microsoft.Extensions.Diagnostics.Testing and dependencies (#6219)

Author: bdovaz

src/Libraries/Microsoft.Extensions.Diagnostics.Testing/Microsoft.Extensions.Diagnostics.Testing.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <RootNamespace>Microsoft.Extensions.Diagnostics.Testing</RootNamespace>
+    <TargetFrameworks>$(NetCoreTargetFrameworks);netstandard2.0;net462</TargetFrameworks>
     <Description>Hand-crafted fakes to make telemetry-related testing easier.</Description>
     <Workstream>Telemetry</Workstream>
     <PackageTags>$(PackageTags);Testing</PackageTags>

src/Libraries/Microsoft.Extensions.Telemetry.Abstractions/Microsoft.Extensions.Telemetry.Abstractions.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <RootNamespace>Microsoft.Extensions.Telemetry</RootNamespace>
+    <TargetFrameworks>$(NetCoreTargetFrameworks);netstandard2.0;net462</TargetFrameworks>
     <Description>Common abstractions for high-level telemetry primitives.</Description>
     <Workstream>Telemetry</Workstream>
     <!-- disable "Published symbols cannot be deleted to maintain compatibility" because we have different APIs for different TFMs -->