How to expose the number of Goroutines as an expvar in Go?

To expose the number of Goroutines as an expvar in Go, you can use the runtime.NumGoroutine() function to get the number of Goroutines at the current moment. This value can then be exposed as an expvar variable.

Here's an example of how you can do this:

package main

import (
	"expvar"
	"net/http"
	"runtime"
)

func main() {
	// Register the expvar variable
	expvar.Publish("goroutines", expvar.Func(func() interface{} {
		return runtime.NumGoroutine()
	}))

	// Start an HTTP server to expose the expvar variables
	http.ListenAndServe(":8080", nil)
}

The above code registers a new expvar variable named "goroutines" whose value is retrieved by calling the runtime.NumGoroutine() function. Then, an HTTP server is started on port 8080 to expose the expvar variables.

To test it, you can run the program and then open http://localhost:8080/debug/vars in your web browser. You should see a JSON response that includes the "goroutines" field with the current number of Goroutines as its value.