How to create and expose a counter variable with expvar in Go?

To create and expose a counter variable with expvar in Go, you can follow these steps:

Step 1: Import the necessary packages:

import (
    "expvar"
    "net/http"
)

Step 2: Declare a global counter variable:

var counter = expvar.NewInt("counter")

Step 3: Implement an HTTP handler to increment the counter:

func incrementCounterHandler(w http.ResponseWriter, r *http.Request) {
    // Increment the counter
    counter.Add(1)
    // Write a response
    w.Write([]byte("Counter incremented"))
}

Step 4: Start the HTTP server and register the /increment endpoint:

func main() {
    http.HandleFunc("/increment", incrementCounterHandler)
    http.ListenAndServe(":8080", nil)
}

Now, when you run your Go program and make a GET request to http://localhost:8080/increment, the counter will be incremented by 1. You can access the counter's current value by visiting http://localhost:8080/debug/vars in your browser. The counter variable will be listed under the expvar variables section.