Merge pull request #12127 from jtattermusch/csharp_warningsaserrors

examples/csharp/helloworld-from-cli/global.json

src/compiler/csharp_generator.cc

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.csproj

src/csharp/Grpc.Core.Tests/ClientServerTest.cs

src/csharp/Grpc.Core.Tests/CompressionTest.cs

src/csharp/Grpc.Core.Tests/ContextPropagationTest.cs

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.csproj

src/csharp/Grpc.Core.Tests/HalfcloseTest.cs

...harp/Grpc.Core.Tests/Internal/CompletionQueueEventTest.cs

src/csharp/Grpc.Core.Tests/Internal/TimespecTest.cs

src/csharp/Grpc.Core.Tests/MockServiceHelper.cs

src/csharp/Grpc.Core.Tests/PerformanceTest.cs

src/csharp/Grpc.Core.Tests/ResponseHeadersTest.cs

src/csharp/Grpc.Core.Tests/ThreadingModelTest.cs

src/csharp/Grpc.Core.Tests/TimeoutsTest.cs

src/csharp/Grpc.Core/Grpc.Core.csproj

src/csharp/Grpc.Core/Internal/AuthContextSafeHandle.cs

.../Grpc.Examples.MathClient/Grpc.Examples.MathClient.csproj

.../Grpc.Examples.MathServer/Grpc.Examples.MathServer.csproj