Prometheus - Components

provides client libraries to instrument services. This will enable you to create and collect custom metrics from your services
has exporters that let you pull metrics that are not in Prometheus format. An exporter acts as a middleman and transforms exported data into Prometheus readable format
provides a powerful query language, PromQL, to work with this collected data
has a GUI that lets you visualize the collected metrics easily. It also integrates with other advanced visualization tools like Grafana

Prometheus - Metric Types

Prometheus offers four core metric types:

counter - represents a single monotonically increasing counter. Its value can increase or reset to zero on restart. You can use it to represent metrics like the number of requests served.
gauge - represents a numerical value that can go up or down. You can use it to represent values like memory usage or the number of requests per second.
histogram - samples data into configurable buckets. Use it to represent values like request durations or response sizes.
summary - similar to a histogram, it also calculates configurable values over a sliding time window