Need to know all the columns in input pipeline using expression in mapper snap

I Want to know the all the input columns in the pipepline  using expression in mapper snap. I used $.keys() function in mapper snap its gives information for all the rows(if we have 1lakh records for processing, its gives information for all 1 lakh records...it leads to performance issue). I need only 1 row output which gives all the column names.

I don't use head,filter snap for this task which makes another line in the pipeline ... to eliminate that we want to write expression in mapper snap.