The fundamental difference between a counter and a gauge in Prometheus lies in their behavior over time. A counter is a cumulative metric that is monotonically increasing; its value can only go up or be reset to zero (typically on a service restart). It is ideal for tracking events like the total number of HTTP requests served. In contrast, a gauge represents a single numerical value that can arbitrarily go up and down. Gauges are suitable for "point-in-time" measurements, such as current memory usage, CPU temperature, or the number of active connections.

A. Both metric types can change between scrapes. Gauges are not static; they represent a current value which can fluctuate frequently.

B. They are two of the four fundamental and distinct metric types in Prometheus, each designed for a different kind of measurement.

C. Both counters and gauges can, and frequently do, use labels to provide dimensionality and context to the metric data.

1. Prometheus Official Documentation, "Metric types":

Counter: "A counter is a cumulative metric that represents a single monotonically increasing counter whose value can only increase or be reset to zero on restart. For example, you can use a counter to represent the number of requests served, tasks completed, or errors."

Gauge: "A gauge is a metric that represents a single numerical value that can arbitrarily go up and down. Gauges are typically used for measured values like temperatures or current memory usage..."

Source: Prometheus Authors. (n.d.). Metric types. Prometheus documentation. Retrieved from https://prometheus.io/docs/concepts/metrictypes/

2. University of Cambridge, Computer Laboratory, "L41: Monitoring and Datalogging":

Section: Prometheus: This course material describes the primary metric types.

Counter: "a value that only ever increases (e.g. number of requests processed)."

Gauge: "a value that can go up or down (e.g. number of requests currently in flight)."

Source: Madhavapeddy, A. (2023). L41: Monitoring and Datalogging. University of Cambridge, Computer Laboratory. Page 11. Retrieved from https://www.cl.cam.ac.uk/teaching/2223/L41/L41-2up.pdf