Prometheus is a full monitoring and trending system that includes built-in and active scraping, storing, querying, graphing, and alerting based on time series data. It has knowledge about what the world should look like (which endpoints should exist, what time series patterns mean trouble, and so on.), and actively tries to find faults.
Prometheus has a rich data model and probably the most powerful query language among time series databases. It encodes dimensions explicitly as key-value pairs (labels) attached to a metric name. That allows easy filtering, grouping, and matching by these labels via in the query language.