Make EventSource.EventMetadata and EventDescriptor public so that analysis roots them.

     These types were public in the separate System.Diagnostics.Tracing assembly, now the move to CoreLib marked them as private.
     While they are technically private (it's a contract used between the library and the ILC toolchain), we need them to be rooted and exported from shared library for the system to work.

     For now I'm simply marking them as public again.
     A cleaner solution might be to use .rd.xml to root them and modify shared library definition to force export them.

[tfs-changeset: 1653635]