Prometheus is a prominent open source tool used for monitoring, and quite a bit of symbiotic work is happening between it and the Kubernetes community. Kubernetes application metrics are exposed in the Prometheus format. This format includes the data types of counter, gauge, histogram, and summary, as well as a means of specifying labels to be associated with specific metrics. As Prometheus and Kubernetes have both evolved, the metrics format from Prometheus appears to be emerging as a de facto standard within the project and across its various components.
More information about this format is available online at the Prometheus project's documentation:
