Package metrics provides minimalist instrumentation for your applications in the form of counters and gauges. A counter is a monotonically-increasing, unsigned, 64-bit integer used to represent the number of times an event has occurred. By tracking the deltas between measurements of a counter over intervals of time, an aggregation layer can derive rates, acceleration, etc. A gauge returns instantaneous measurements of something using signed, 64-bit integers. This value does not need to be monotonic. A histogram tracks the distribution of a stream of values (e.g. the number of milliseconds it takes to handle requests), adding gauges for the values at meaningful quantiles: 50th, 75th, 90th, 95th, 99th, 99.9th. Measurements from counters and gauges are available as expvars. Your service should return its expvars from an HTTP endpoint (i.e., /debug/vars) as a JSON object.