Description of problem:

The issue with extracting reliable usage and event data from openstack with gnocchi.


The gnocchi resource "started_at" timestamps are not the correct timestamps of the nova instance start time, as they differ a few minutes from the nova data and the ceilometer event data for the instance start event. 
The started_at is set the first time that Ceilometer creates the resource in Gnocchi, no data from Nova is used.
Gnocchi resources never get an “ended_at” date.

Version-Release number of selected component (if applicable):
Gnocchi/Ceilometer in OSP10


How reproducible:

The gnocchi resource of type instance has the following timestamp:
(gnocchi resource list or gnocchi resource show)

Resource started at:
2017-12-01T10:11:34.724926+00:00
Revision started at:
2017-12-01T10:11:34.724948+00:00 


nova show delivers the following for the launched_at param of nova:
OS-SRV-USG:launched_at               | 2017-12-01T10:07:36.000000  

When looking up the ceilometer compute.instance.create.end event, this has the timestamp:
 ceilometer event-list -q 'event_type=compute.instance.create.end;instance_id=5023b71f-c8c2-4a1a-82f0-1fdafdb090b7'

 launched_at    | datetime |                    2017-12-01T10:07:36.759000      
The according ceilometer event was generated at: 2017-12-01T10:07:37.017000


Actual results:
Gnocchi started_at timestamps are not in sync with the timestamps of the ceilometer events.
There are nearly 4 minutes in between the launched_at timestamp of the instance and the "started_at" timestamp of gnocchi.

Gnocchi ended_at timestamps are missing.

There is also (small) difference in between the launched_at time value in nova and the ceilometer event.


Expected results:
Customer needs reliable data for metering and billing.
At the moment It looks like every component creates an own timestamp.