question on audit columns

In postgres database , we have all the tables with audit columns like created_by_user, created_timestamp,updated_by_user, updated_timestamp. So we have these fields that were supposed to be populated by the time at which the insert/update operation happened on the database but not at the application level. So we are planning to populate the created_by_user, created_timestamp columns by setting a default value of "current_timestamp" and "current_user" for the two columns,  but no such this is available to populate while we do the update of the row, so the only option seems to be through a trigger. 

So wanted to check with the experts here  ,considering the table will be DML heavy table (300M+ transactions will be inserted daily), Is is okay to have the trigger for this table for populating all the audit columns or should we keep default for  created_by_user, created_timestamp and just trigger for the update related two audit column? Basically wanted to see, if the default value does the same thing as a trigger or it does something more optimally than trigger?

Regards

Yudhi