// Change alert count to something reasonable for production
var alertCountLimit = 100;
// Field name from additional_info to set as metric name
var newMetricName = 'u_field_you_want';

var alertGr = new GlideRecord('em_alert');
alertGr.setLimit(alertCountLimit);
alertGr.query();

while (alertGr.next()) {
	if (alertGr.additional_info != '') {
		if (typeof JSON.parse(alertGr.additional_info)[newMetricName]) != 'undefined') {
		    alertGr.metric_name = JSON.parse(alertGr.additional_info)[newMetricName]
		    gs.info('***metric name population***\n Changed ' + alertGr.number + ' metric_name to' + JSON.parse(alertGr.additional_info)[newMetricName]);
		}
    	}
}