I think installation of collectd should create a unix group called "collectd". Especially when installed with unixsock plugin (unixsock chowns the unix socket file to this group). Part of collectd-unixsock manual: SocketGroup Group - if running as root change the group of the UNIX-socket after it has been created. Defaults to collectd.
Thanks for the suggestion -- assigning to maintainer.
Done in -r4 - thanks!