Unify the timestamp of samples generated by polling agents

Many of the pollsters define the timestamp individually for each generated
sample, which should really be timestamping based on when the data was polled
and not when each sample is generated. We need to set the timestamp of the
polled data to the timestamp when the polling starts for unity.

Related-Bug: #1491509
Blueprint unify-timestamp-of-polled-data

Change-Id: If22980c9fdf81e1c76dcd4a9053daf88d7f89d18

specs/mitaka/unify-timestamp-of-polled-data.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+============================================================
+Unify the timestamp of samples generated by polling agents
+============================================================
+
+https://blueprints.launchpad.net/ceilometer/+spec/unify-timestamp-of-polled-data
+
+
+Problem description
+===================
+
+Many of the pollsters define the timestamp individually for each generated
+sample, which should really be timestamping based on when the data was polled
+and not when each sample is generated. We need to set the timestamp of the
+polled data to the timestamp when the polling starts for unity.
+
+This should save overhead of calculating timestamp each sample and also allow
+for easier grouping of polled data.
+
+Proposed change
+===============
+
+Modify ceilometer.agent.manager.PollingTask.poll_and_notify method to set the
+timestamp of samples to the time when the data was polled::
+
+ # Get the timestamp when pollster starts to work
+ polling_timestamp = self._get_current_timestamp()
+
+ samples = pollster.obj.get_samples(
+     manager=self.manager,
+     cache=cache,
+     resources=polling_resources
+ )
+
+ for sample in samples:
+ # Unify the timestamp of polled samples
+ sample.set_timestamp(polling_timestamp)
+
+Drop all the timestamp generations when we build samples via pollsters, like:
+https://github.com/openstack/ceilometer/blob/6.0.0.0b2/ceilometer/image/glance.py#L113
+
+Alternatives
+------------
+
+None
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Pipeline impact
+---------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance/Scalability Impacts
+-------------------------------
+
+Save overhead of calculating timestamp each sample.
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  <yuywz>
+
+
+Work Items
+----------
+
+Update ceilometer.agent.manager.PollingTask.poll_and_notify method.
+
+Drop all the timestamp generations when we build samples via pollsters.
+
+
+Future lifecycle
+================
+
+None
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Add unit test to test:
+The timestamp of samples generated during a polling cycle is unified.
+
+
+Documentation Impact
+====================
+
+None
+
+
+References
+==========
+
+Originally reported as a bug: https://bugs.launchpad.net/ceilometer/+bug/1491509
+Blueprint: https://blueprints.launchpad.net/ceilometer/+spec/unify-timestamp-of-polled-data