Prometheus doesn’t have a good built in story for horizontal scaling of storage and queries, or long term storage. That’s why there’s M3DB, Cortex, etc. which let you send metrics to a set of Prometheus servers which then write to a cluster that handles broad queries and tiered storage. So these are less a competitor to Prometheus and Grafana and more an augmentation of it since it supports both emitting metrics in Prometheus form and querying them with PromQL.

Datadog gets very expensive beyond a certain point; much bigger than most startups, but much smaller than Uber.

Cloudwatch is usually used as a source of metrics rather than a destination. It doesn’t have as rich a data model as Prometheus for custom metrics, and has a lot of quite limiting restrictions (like only 10 tags per metric).