Prometheus is a monitoring system and time series database that enables scraping of metrics from multiple targets, storing and querying those metrics in tools like Grafana and also managing alerts based on threshold for metrics. It is a fundamental service in Observability stacks.
Prometheus is not designed for horizontally scaling, and this can impose some limitations for large scale setups. VictoriaMetrics is an interesting alternative that promises a better use of resources and better scalability.