Which Prometheus metric represents a single value that can go up and down?

A Counter is a cumulative metric that represents a single monotonically increasing counter whose value can only increase or be reset to zero upon restart, unlike a Gauge.

A Gauge is a metric that represents a single numerical value that can go up and down arbitrarily, making it suitable for continuously changing values such as current memory usage, temperature, or the number of concurrent requests.

A Summary samples observations (usually request durations or response sizes) and provides quantiles over a sliding time window, rather than a single fluctuating value.

A Histogram samples observations and counts them in configurable buckets, also providing a sum of all observed values and an event count, which is different from a single, arbitrarily changing value.