How to create and expose custom runtime variables using expvar in Go?

To create and expose custom runtime variables using expvar in Go, you can follow these steps:

Import the required packages:

import (
	"expvar"
	"net/http"
)

Define global variables that you want to expose:

var (
	totalRequests = expvar.NewInt("total_requests")
	responseTime  = expvar.NewFloat("response_time")
)

Increment and update the values of these variables wherever necessary in your code:

totalRequests.Add(1)
responseTime.Set(0.123)

Optionally, create an HTTP handler function to expose these variables:

func expvarHandler(w http.ResponseWriter, r *http.Request) {
	expvar.Do(func(kv expvar.KeyValue) {
		// Write the key-value pairs to the response
		fmt.Fprintf(w, "%s: %s\n", kv.Key, kv.Value)
	})
}

http.HandleFunc("/debug/vars", expvarHandler)

Start the HTTP server to listen and serve requests:

http.ListenAndServe(":8080", nil)

Now, when you access http://localhost:8080/debug/vars in your web browser or through a tool like cURL, you will see a list of all variables and their values, including the custom runtime variables defined using expvar.

Note: expvar package does not provide any form of authentication or security. So, make sure to handle the access to these runtime variables appropriately in production systems.