Previouslty "Size" was reserved by protoc-gen-gogoctrd and user-generated
"Size" was automatically renamed to "Size_" to avoid conflicts.
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>
gogoproto.customtype is used to have go-digest.Digest instead of string.
While it is convinient, protoc-gen-go doesn't support the extension
and that blocks #6564.
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>
