Prometheus is an open-source systems monitoring and alerting toolkit, originally built at SoundCloud in 2012. It stores metrics as time series with a name and optional labels (a multi-dimensional data model) and provides the PromQL query language. The Prometheus server pulls metrics over HTTP from targets that are statically configured or found through service discovery; a Pushgateway exists for short-lived jobs. There are also client libraries for instrumenting application code, exporters for services such as HAProxy and StatsD, and Alertmanager for handling alerts. Each server operates on its own, without distributed storage.
Prometheus is independent of any single company: the project joined the Cloud Native Computing Foundation in 2016, as its second hosted project after Kubernetes. All components are available under the Apache 2.0 licence. Because you run Prometheus yourself, the monitoring data stays on the infrastructure you choose, and you decide the location and jurisdiction. For a European organisation, self-hosting on its own or European infrastructure is therefore a direct way to keep monitoring data under its own control.