How to integrate expvar with a monitoring and alerting system like Prometheus in Go?

To integrate expvar with Prometheus in Go, you can follow these steps:

Add the necessary packages to your project by running the following command:

go get github.com/prometheus/client_golang/prometheus

Import the required packages in your Go file:

import (
	"net/http"
	_ "net/http/pprof"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

Define your expvar variables and collect them using Prometheus by creating a custom collector. For example:

// Define your expvar variables
var (
	totalRequests = expvar.NewInt("total_requests")
	successfulRequests = expvar.NewInt("successful_requests")
)

// Custom collector to collect the expvar variables
type CustomCollector struct {
	totalRequests prometheus.Gauge
	successfulRequests prometheus.Gauge
}

// Describe method to describe the metrics
func (c *CustomCollector) Describe(ch chan<- *prometheus.Desc) {
	ch <- c.totalRequests.Desc()
	ch <- c.successfulRequests.Desc()
}

// Collect method to collect the metrics
func (c *CustomCollector) Collect(ch chan<- prometheus.Metric) {
	ch <- c.totalRequests
	ch <- c.successfulRequests
}

// Register the collector
func init() {
	// Register the custom collector
	prometheus.Register(&CustomCollector{
		totalRequests: prometheus.NewGauge(prometheus.GaugeOpts{
			Name: "total_requests",
			Help: "Total number of requests",
		}),
		successfulRequests: prometheus.NewGauge(prometheus.GaugeOpts{
			Name: "successful_requests",
			Help: "Total number of successful requests",
		}),
	})
}

// Update the expvar variables
func updateExpvarMetrics() {
	// Update the expvar variables
	totalRequests.Add(1)
	successfulRequests.Add(1)
}

Start the Prometheus metrics endpoint in your application:

// Start the Prometheus metrics endpoint
func startPrometheusMetrics() {
	// Start the Prometheus endpoint
	http.Handle("/metrics", promhttp.Handler())
	go func() {
		if err := http.ListenAndServe(":8080", nil); err != nil {
			log.Fatal("Error starting Prometheus endpoint: ", err)
		}
	}()
}

Call the updateExpvarMetrics function whenever you want to update the expvar variables.
Start the Prometheus metrics endpoint by calling the startPrometheusMetrics function at the beginning of your application.

Now, you can access your expvar variables in Prometheus by scraping the /metrics endpoint of your Go application.