add profiling support

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

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

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

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

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

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

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

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

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

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

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

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

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

src/csharp/Grpc.Core/Profiling/IProfiler.cs

src/csharp/Grpc.Core/Profiling/ProfilerEntry.cs

src/csharp/Grpc.Core/Profiling/ProfilerScope.cs

src/csharp/Grpc.Core/Profiling/Profilers.cs