Fix alert latency metric
Fix alert latency metric

mrkeng/alerta,guardian/alerta,skob/alerta,skob/alerta,mrkeng/alerta,mrkeng/alerta,0312birdzhang/alerta,0312birdzhang/alerta,mrkeng/alerta,skob/alerta,0312birdzhang/alerta,guardian/alerta,guardian/alerta,skob/alerta,guardian/alerta