presto-orc/src/main/java/io/prestosql/orc/metadata/OrcMetadataReader.java/OrcMetadataReader/toUserMetadata
class OrcMetadataReader:    private static Map<String, Slice> toUserMetadata(List<OrcProto.UserMetadataItem> metadataList)
    {
        ImmutableMap.Builder<String, Slice> mapBuilder = ImmutableMap.builder();
        for (OrcProto.UserMetadataItem item : metadataList) {
            mapBuilder.put(item.getName(), byteStringToSlice(item.getValue()));
        }
        return mapBuilder.build();
    }