To append a line using sed that works on macOS:

sed -e '/^telegraf_enab.*/a\'$'\n''telegraf_user="root"'

If need to do it in multiple files, you could use find:

 find . -name "rc.conf" -exec sed -i '' -e '/^telegraf_enab.*/a\'$'\n''telegraf_user="root"' {} \;

https://stackoverflow.com/a/48406504/1135424