capture the timestamp for the history of customized attributes in RRDI

I may not have fully understood your question, but the data model does not store a timestamp with each custom attribute.  If you want to know when a particular custom attribute was changed, I think you need to join the row containing your custom attribute from the appropriate table based on type (for example REQUEST_HISTORY_STRING_EXT) to the REQUEST_HISTORY table via REQUEST_HISTORY_ID and use the REQUEST_HISTORY.REC_DATETIME value.