Add .NET Standard 2.1 as a target (#22)

Adding .NET standard 2.1 as a target. This enables using prometheus-net.DotNetStats in projects built for .NET core 2.1 but running on .NET core 2.2+.

Author: blankensteiner

src/prometheus-net.DotNetRuntime/prometheus-net.DotNetRuntime.csproj

@@ -24,12 +24,16 @@
         <PackageLicense>https://github.com/djluck/prometheus-net.DotNetRuntime/blob/master/LICENSE.txt</PackageLicense>
         <Configurations>ReleaseV2;DebugV2;DebugV3;ReleaseV3</Configurations>
         <Platforms>AnyCPU</Platforms>
-        <TargetFrameworks>netcoreapp2.2;netcoreapp3.0</TargetFrameworks>
+        <TargetFrameworks>netcoreapp2.2;netcoreapp3.0;netstandard2.1</TargetFrameworks>
         <LangVersion>8</LangVersion>
         <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
         <NoWarn>1701;1702;CS1591;</NoWarn>
     </PropertyGroup>
 
+    <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.1'">
+        <PackageReference Include="System.Collections.Immutable" Version="1.6.0" />
+    </ItemGroup>
+
     <PropertyGroup Condition=" $(Configuration.StartsWith('Release')) ">
         <Optimize Condition=" '$(Optimize)' == '' ">true</Optimize>
     </PropertyGroup>