Add System.Diagnostics.DiagnosticSource reference for NetStandard 2.0 (#3948)

Fixes #3945

(cherry picked from commit 44e9836)

Author: russcam

build/Elasticsearch.Net.nuspec

@@ -11,13 +11,14 @@
 		<iconUrl>https://raw.githubusercontent.com/elastic/elasticsearch-net/master/build/nuget-icon.png</iconUrl>
 		<requireLicenseAcceptance>false</requireLicenseAcceptance>
 		<description>Elasticsearch.Net</description>
-		<summary>Exposes all the elasticsearch API endpoints but leaves you in control of building the request and response bodies. Comes with built in cluster failover/connection pooling support.</summary>
+		<summary>Exposes all the Elasticsearch API endpoints but leaves you in control of building the request and response bodies. Comes with built in cluster failover/connection pooling support.</summary>
 		<releaseNotes>See https://github.com/elastic/elasticsearch-net/releases/tag/$version$</releaseNotes>
 		<copyright>2014-$year$ Elasticsearch BV</copyright>
 		<tags>elasticsearch,elastic,search,lucene,nest</tags>
 		<dependencies>
 			<group targetFramework=".NETStandard2.0">
 				<dependency id="System.Buffers" version="[4.5.0,)"/>
+				<dependency id="System.Diagnostics.DiagnosticSource" version="[4.5.1,)" />
 				<dependency id="System.Reflection.Emit" version="[4.3.0,)"/>
 				<dependency id="System.Reflection.Emit.Lightweight" version="[4.3.0,)"/>
 			</group>

src/Elasticsearch.Net/Elasticsearch.Net.csproj

@@ -20,7 +20,6 @@
 
     <PackageReference Condition="'$(TargetFramework)' == 'netstandard2.0'" Include="System.Reflection.Emit" Version="4.3.0" />
     <PackageReference Condition="'$(TargetFramework)' == 'netstandard2.0'" Include="System.Reflection.Emit.Lightweight" Version="4.3.0" />
-    
   </ItemGroup>
   <ItemGroup>
     <Compile Update="ElasticLowLevelClient.*.cs">