Merge pull request #2459 from jtattermusch/csharp_metadata_refactoring

src/compiler/csharp_generator.cc

src/csharp/Grpc.Auth/OAuth2InterceptorFactory.cs

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

src/csharp/Grpc.Core/Calls.cs

src/csharp/Grpc.Core/ClientBase.cs

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

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

src/csharp/Grpc.Core/Metadata.cs

src/csharp/Grpc.Core/Stub/StubConfiguration.cs

src/csharp/Grpc.Core/Version.cs

src/csharp/Grpc.Examples.MathClient/MathClient.cs

src/csharp/Grpc.Examples.Tests/MathClientServerTests.cs

src/csharp/Grpc.Examples/MathExamples.cs

src/csharp/Grpc.Examples/MathGrpc.cs

src/csharp/Grpc.HealthCheck.Tests/HealthClientServerTest.cs

src/csharp/Grpc.HealthCheck/HealthGrpc.cs

src/csharp/Grpc.IntegrationTesting/InteropClient.cs

...csharp/Grpc.IntegrationTesting/InteropClientServerTest.cs

src/csharp/Grpc.IntegrationTesting/TestGrpc.cs

src/csharp/generate_proto_csharp.sh