Bug 2215 fixed

- created an attribute on the Alarm class, extra_labels
- create a method on common_db to include new extra_labels
- changed the _get_metric_value_from_response method of Prometheus Evaluator to return both the metric value and labels

Change-Id: I22e0fd8e95de28560e3a3216622c43c8861137ef
Signed-off-by: Daniel Gomes <dagomes@av.it.pt>