With CF Top plugin there are two fields "LOG_OUT" and  "LOG_ERR" which gives the total logged messages since cf top has been running.   Although it doesn't give you a rate of messages per minute, it can quickly identify chatty apps. If you have "doppler.firehose" scope it will show logging totals for all apps, if not it will only show apps you have authority to see.   You can sort by any column, so sorting on "LOG_OUT" column will show the top loggers.