From looking at a recent undercloud with OSP13 installed, it seems to be included already.
Verified. /usr/share/openstack-tripleo-heat-templates/puppet/services/metrics/collectd.yaml includes Gnocchi code.
tested that collectd-gnocchi deployed by default by overcloud deployment and that metreics from the plugins are coming to the internal gnocchi instance.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.