Explain what the Counter metric means in the Prometheus monitoring system.

In Prometheus, a Counter metric is a counter that only increases or resets to zero. It is used to count the number of events, such as server requests, errors, or processed messages.

Main properties of Counter:

• The value does not decrease (only increases or resets).
• Used for accumulating total data.

Example of usage in Go with the Prometheus client library:

var opsProcessed = prometheus.NewCounter(prometheus.CounterOpts{
    Name: "myapp_processed_ops_total",
    Help: "The total number of processed events",
})

func process() {
    // event processing
    opsProcessed.Inc() // increment the counter by 1
}

Thus, Counter helps track the total number of events that occurred during the application's runtime.