Returns an array of supported processed metrics and their corresponding translations. Eg
array('nb_visits' => 'Visits'). By default the given {@link $processedMetrics} are used and their
corresponding translations are looked up automatically. If a metric is not translated, you should add the
default metric translation for this metric using the {@hook Metrics.getDefaultMetricTranslations} event. If you
want to overwrite any default metric translation you should overwrite this method, call this parent method to
get all default translations and overwrite any custom metric translations.