Merge pull request #2597 from jtattermusch/csharp_metadata

src/compiler/csharp_generator.cc

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

...p/Grpc.Core.Tests/Internal/MetadataArraySafeHandleTest.cs

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

src/csharp/Grpc.Core/AsyncClientStreamingCall.cs

src/csharp/Grpc.Core/AsyncDuplexStreamingCall.cs

src/csharp/Grpc.Core/AsyncServerStreamingCall.cs

src/csharp/Grpc.Core/AsyncUnaryCall.cs

src/csharp/Grpc.Core/Calls.cs

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

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

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

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

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

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

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

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

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

src/csharp/Grpc.Core/Metadata.cs

src/csharp/Grpc.Core/Server.cs