Prometheus, started by SoundCloud in 2012, is a CNCF-graduated open-source monitoring and alerting system that uses a Pull model with built-in PromQL - the de-facto monitoring standard for cloud-native and Kubernetes environments.
Grafana, created by Grafana Labs in 2014 (Stockholm/New York), is the world's most popular open-source monitoring visualization platform with 20M+ users, 100+ data sources and 30+ panel types.
Thanos is a cloud-native open-source project providing highly available Prometheus setups and long-term storage, using a Sidecar architecture, and became a CNCF graduated project in 2021, used by thousands of enterprises for production observability.