Duplicate data sent in samples for network delta metrics

[Arnaud Morin]

When using a pollsters that use the same inspector, ceilometer caches the first results collected from inspector (thanks to _inspect_cached) so that it can be reused later when collecting other metrics from the same inspector.

This was introduced in a refactor commit when fixing [1].

While it's good for most of the stats collected by ceilometer, it has a side effect on delta metrics which is wrong.

E.G. if you enable the polling of both network.outgoing.bytes.delta and network.incoming.bytes.delta, then the inspector will be collected only once (because the second one is red from cache).

On a later collecting job, due to the fact that we shuffle randomly the samples to collect (see [2]), we may use another collector for the same metrics, but the delta will then be wrong because it was not updated on this collector on the previous run!

[1] https://bugs.launchpad.net/ceilometer/+bug/1670948
[2] https://github.com/openstack/ceilometer/blob/master/ceilometer/polling/manager.py#L167