Dynamically enable or disable meters

Currently there is no way to enable or disable meters without restarting ceilometer. There are cases where operators do not want to run all the meters continuously.

With this blueprint, it will be possible to enable or disable meters without restart of ceilometer agents. Changes needs to be done in ceilometer-collector, ceilometer-central agent, and ceilometer-compute agent.

Here is a high level call flow.
Using “pyinotify” monitor the pipeline.yaml for changes. This can be done by a stand-alone process or a script. When the file is modified, signal is sent to ceilometer-collector, ceilometer-central, ceilometer-compute agents. The signal handlers gets invoked in the ceilometer agents.

Then the ceilometer agents shall:
1. Read the yaml file
2. Iterate through the list of polling tasks objects
    a. In the yaml file check if the meter is added/deleted under the respective pipelines in polling_task
    b. If meter is added then update the list of PollingTask object with the corresponding pollster object.
    c. if meter is deleted then delete the pollster object from list of pollsters in PollingTask object.