Add human readable displayversion to dashboard metrics

[sqwk]

At the moment metrics can only be formatted using Intl.NumberFormat. This falls short when displaying for example durations. Is it possible to add a human readable display (string) version that is used for non-graphs?

For example:

$this->registerMetric(new ReportMetric(
    self::METRIC_ENTRY_DURATION,
    'TIMESTAMPDIFF(MINUTE, start_time, end_time)',
    'Recorded Entry Duration',
    ReportMetric::AGGREGATE_SUM,
    null, // Intl.NumberFormat would normally go here
    function ($value) {
        $totalMinutes = $value;
        $hours = intdiv($totalMinutes, 60);
        $minutes = $totalMinutes % 60;
        return sprintf('%d:%dh', $hours, $minutes);
    }
));

(I am aware that TIMESTAMPDIFF does currently not work. See #5924 and #5966)