A gauge is the correct Prometheus metric type for values that can arbitrarily go up and down. A Unix timestamp, which represents a point in time, fits this definition perfectly. It is a common and recommended practice to export the timestamp of a recent event (e.g., last successful backup, last data processing job) as the value of a gauge. This allows for easy monitoring and alerting, for instance, by checking if the timestamp is too old (time() - myapplastjobtimestampseconds > DURATION). This approach is semantically correct and efficient.

B: Overriding the metric timestamp is a specialized feature for exporters that proxy metrics from other systems and is not the standard way to expose an application-level timestamp.

C: A counter is a cumulative metric that only increases. It is semantically incorrect for a timestamp, as you cannot set a counter to an arbitrary value.

D: Using a timestamp as a label value is a critical anti-pattern. It would create a new time series for every unique timestamp, leading to a high-cardinality problem that can severely degrade Prometheus's performance.

1. Prometheus Official Documentation, Metric Types: The documentation defines a gauge as "a metric that represents a single numerical value that can arbitrarily go up and down." It lists examples like temperatures or memory usage, which are analogous to a timestamp value that is updated periodically.

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

2. Prometheus Official Documentation, Naming and Labels Best Practices: This official guide explicitly warns against using labels for high-cardinality values. "Do not use labels to store dimensions with high cardinality (many different label values), such as user IDs, email addresses, or other unbounded sets of values." A timestamp is an unbounded value, making it unsuitable for a label.

Source: Prometheus Authors. (n.d.). Metric and label naming. Prometheus documentation. Retrieved from https://prometheus.io/docs/practices/naming/#labels

3. University of California, Berkeley, CS 162 Courseware: In lecture materials on monitoring, a gauge is described as a metric whose "Value can go up and down." A specific example provided for a gauge's use is the "last time something happened," which directly corresponds to exposing a timestamp.

Source: Kubiatowicz, J., & Culler, D. (2021). Lecture 24: Monitoring. CS162: Operating Systems and System Programming, UC Berkeley. Page 27. Retrieved from https://cs162.eecs.berkeley.edu/sp21/static/lectures/24-monitoring.pdf