The ping plugin uses "value" dataset name, not "ping"

Using collectd 5.4.1, the RRD files are produced with DS name "value", not "ping". Looking at collectd's git lof of src/ping.c, I was not able to find out when this got changed.

Random googling suggests that this was reported back in 2012 via https://groups.google.com/forum/#!msg/collectd-web-users/S6bAC-L5FT0/9nuHhw6kvT0J .