A gauge is a metrics type that holds or return a single value (numeric or textual) where changes can only be observed in random order (increasing or decreasing order)
If the single value only increase or decrease, see:
Gauge values are not rates.
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A gauge usually plots one data point with an indication of whether that point falls in an acceptable or unacceptable range.
Gauges are useful for monitoring things with natural upper bounds such as Resources metrics.
As a gauge shows only the current value, the value of the gauge should only be fetched at publish time.