Is there any way to enable serialization of underscore prefixed fields by to_json or xm_gelf

Hi,

I'm trying to fit output into the GELF format and I'd like to preserve their specification that user fields have underscore prefixes. From what I read in the nxlog docs, any fields with underscore prefix wouldn't be preserved by xm_json or xm_gelf

Is that true?

Is there any way around this?