Track the status of the pollsters

Currently the Ceilometer agents will create a polling task for each different source interval defined in the pipeline configurations. All the pollsters from different sources with the same interval will be run in that polling task which in turn is running in a single green thread.

In the case of many pollsters with the same small interval value, it's quite likely that whole time to process all the pollsters is way longer than the interval itself. Besides, it is also possible that a specific pollster is taking a much longer time than others due to unexpected reasons(i.e. network connecting issue, peer service responsiveness, etc). Thant's why we have the bp https://blueprints.launchpad.net/ceilometer/+spec/central-agent-improvement to make the agent scale out.

But here we may also need to track the pollsters' status in the PollingTask, this could help the admin to find out whether the agent's task is overloaded by some specific pollsters.