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.
