The correct option is Alertmanager.

The Alertmanager receives alerts from Prometheus servers and it coordinates grouping, silencing, routing and delivery of those alerts. It sends notifications to administrators and external channels such as email, Slack and webhook endpoints according to configured routes and receivers.

Cloud Monitoring is a managed cloud provider monitoring service and not the Prometheus component responsible for routing or delivering alerts to notification channels.

Pushgateway is used for pushing metrics from short lived jobs into Prometheus and it does not handle alert processing or notification delivery.

Exporters expose application or system metrics for Prometheus to scrape and they do not perform alert handling or send notifications.

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