Instrumenting services using gauges

Gauges are very similar to counters. The only significant difference is that we can not only increment, but also decrease their values. We'll continue exploring vfarcic/docker-flow-swarm-listener GitHub repository for an example of a gauge.

Since gauge is almost identical to counter, we won't go into many details but only briefly explore a few snippets.

Just as with a counter, we need to declare a variable that defines the type of the metric. A simple example is as follows:

var serviceGauge = prometheus.NewGaugeVec(
    prometheus.GaugeOpts{
       Subsystem: "docker_flow",
        Name: "service_count",
        Help: "Service gauge",
    },
    []string{"service"},
)

Next, we need to register it with Prometheus. We'll reuse the code from the init function where we defined the errorCounter and add serviceGauge.

func init() {
    prometheus.MustRegister(errorCounter, serviceGauge)
}

There's also a function that simplifies the usage of the metric:

func RecordService...