Revert "Revert "Metadata handling rewrite""

This reverts commit 5e01e2ac977655aa074faf7fde0a74298f5e4c55.
diff --git a/tools/doxygen/Doxyfile.c++.internal b/tools/doxygen/Doxyfile.c++.internal
index 2b2d8a3..5bd4323 100644
--- a/tools/doxygen/Doxyfile.c++.internal
+++ b/tools/doxygen/Doxyfile.c++.internal
@@ -823,6 +823,7 @@
 include/grpc++/impl/codegen/server_context.h \
 include/grpc++/impl/codegen/server_interface.h \
 include/grpc++/impl/codegen/service_type.h \
+include/grpc++/impl/codegen/slice.h \
 include/grpc++/impl/codegen/status.h \
 include/grpc++/impl/codegen/status_code_enum.h \
 include/grpc++/impl/codegen/status_helper.h \