How to integrate expvar with existing instrumentation and monitoring tools in Go?

Integrating expvar with existing instrumentation and monitoring tools in Go can be done by exposing the expvar metrics and making them available to these tools. Here's an example of how you can approach this:

Import the necessary packages:

import (
	"expvar"
	"net/http"
)

Create an expvar.Map to hold your custom expvar metrics:

var myMetrics = expvar.NewMap("myapp")

func init() {
	myMetrics.Add("counter", new(expvar.Int))
	myMetrics.Add("timer", new(expvar.Int))
}

Update your code where the relevant metrics are updated:

func someFunction() {
	// Increment the counter metric
	myMetrics.Get("counter").(*expvar.Int).Add(1)

	// Measure the execution time
	start := time.Now()

	// ... code execution

	// Update the timer metric
	myMetrics.Get("timer").(*expvar.Int).Add(int64(time.Since(start).Milliseconds()))
}

Expose the /debug/vars endpoint to make the expvar metrics available:

http.Handle("/debug/vars", expvar.Handler())

Start the HTTP server:

go http.ListenAndServe(":8080", nil)

Now, your expvar metrics are available at http://localhost:8080/debug/vars. You can integrate them with your existing instrumentation and monitoring tools by configuring them to scrape the metrics from this endpoint.

Note that you can also choose to expose the expvar metrics over other protocols like Prometheus, StatsD, or Graphite, depending on the tools you use. In that case, you'll need to use libraries or exporters compatible with those protocols to expose the expvar metrics accordingly.