If you want to monitor the uptime of an PPP connection, you can easily use the ctime of the PID file. To get the time into collectd, a python module is the best choice:

import collectd 
import os
import time

def reader_calback():
    v2 = collectd.Values(type='uptime') 
    v2.plugin='pppoe'
    v2.type_instance='Uptime' 
    v2.dispatch(values=[time.time() - os.stat("/var/run/ppp0.pid").st_ctime]) # Value to be dispatched


collectd.register_read(reader_calback)