A metric metadata class is a class that describes how a metric is described, computed and
formatted.
There are two types of metrics: aggregated and processed. An aggregated metric is computed
in the backend datastore and aggregated in PHP when archiving period reports.
Currently, only processed metrics can be defined as metric metadata classes. Support for
aggregated metrics will be added at a later date.
See {@link Piwik\Plugin\ProcessedMetric} and {@link Piwik\Plugin|AggregatedMetric}.