Merge pull request #3468 from jtattermusch/csharp_metadata_plugin

src/csharp/Grpc.Auth/AuthInterceptors.cs

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

src/csharp/Grpc.Auth/GrpcCredentials.cs

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

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

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

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

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

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

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

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

src/csharp/Grpc.Core/CallCredentials.cs

src/csharp/Grpc.Core/CallOptions.cs

src/csharp/Grpc.Core/Channel.cs

src/csharp/Grpc.Core/ChannelCredentials.cs

src/csharp/Grpc.Core/ClientBase.cs

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

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

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

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