Prometheus is a free software application used for event monitoring and alerting. It records real-time metrics in a time series database built using a HTTP pull model, with flexible queries and real-time alerting.
Alertmanager is the service (and software) in charge of collecting, de-duplicating and sending notifications for alerts across WMF infrastructure. It is part of the Prometheus ecosystem and therefore Prometheus itself has native support to act as Alertmanager client. The alerts dashboard, implemented by Karma, can be reached at https://alerts.wikimedia.org/