This refactors common metrics importer for two reasons. 1. To create a
new pattern that can be followed by future importers that will minimize
dependency collision and 2. To allow EE to more easily extend
dependencies.

Correct the ordering of metrics on performance dashboard. Before common
metrics were moved into the DB, metric groups were ordered by the
priority defined in the common_metrics.yml file.
This commit adds a priority to each metric group in the PrometheusMetric
model.
It also combines title, priority and required_metrics into one frozen
GROUP_DETAILS hash so that the code is clearer.
This can be done since there is a fixed set of groups which are not
configurable.

This MR backports PrometheusMetric model to CE
and adds: common, identifier to figure out what kind of metric is used.