How to expose custom application-specific metrics with expvar in Go?

To expose custom application-specific metrics with expvar in Go, you can follow these steps:

Start by importing the necessary packages:

import (
	"expvar"
	"net/http"
)

Create a struct to hold your custom metrics:

type customMetrics struct {
	Counter    expvar.Int
	Average    expvar.Float
}

Initialize your custom metrics in the init() function:

func init() {
	expvar.Publish("CustomMetrics", expvar.Func(func() interface{} {
		return customMetrics{
			Counter:    expvar.NewInt("counter"),
			Average:    expvar.NewFloat("average"),
		}
	}))
}

Increment and update your custom metrics whenever necessary:

// Increment counter metric
customMetrics.Counter.Add(1)

// Update average metric
value := 10.5
customMetrics.Average.Update(value)

Note that the customMetrics struct is published as a JSON object under the /debug/vars endpoint. You can access it by visiting http://localhost:port/debug/vars in your browser.

Finally, start an HTTP server to listen for requests:

func main() {
	http.ListenAndServe(":8080", nil)
}

Now your custom metrics will be exposed alongside the default expvar metrics. You can use expvar for monitoring and exposing various metrics and runtime variables in your Go application.