```golang
func main() {
	router := gin.Default()

	// Register /metrics before middleware
	router.GET("/metrics", PrometheusHandler())
	
	router.Use(RequestMetricsMiddleware())
	router.GET("/health", func(c *gin.Context) {
		c.JSON(200, gin.H{
			"message": "Up and running!",
		})
	})
	router.GET("/v1/users", func(c *gin.Context) {
		c.JSON(200, gin.H{
			"message": "Hello from /v1/users",
		})
	})

	router.Run(":8000")
}
```

Now comes the middleware function `RequestMetricsMiddleware`. This function is called for every request made to the API. It increments the `HttpRequestTotal` counter (different counter for different paths and status codes) if the status code is less than or equal to 400. If the status code is greater than 400, it increments the `HttpRequestErrorTotal` counter (different counter for different paths and status codes). The `PrometheusHandler` function is the custom handler that will be called for the `/metrics` endpoint. It will return the metrics in the Prometheus format.

```golang
// Custom metrics handler with custom registry
func PrometheusHandler() gin.HandlerFunc {
	h := promhttp.HandlerFor(customRegistry, promhttp.HandlerOpts{})
	return func(c *gin.Context) {
		h.ServeHTTP(c.Writer, c.Request)
	}
}

// Middleware to record incoming requests metrics
func RequestMetricsMiddleware() gin.HandlerFunc {
	return func(c *gin.Context) {
		path := c.Request.URL.Path
		c.Next()
		status := c.Writer.Status()
		if status < 400 {
			HttpRequestTotal.WithLabelValues(path, strconv.Itoa(status)).Inc()
		} else {
			HttpRequestErrorTotal.WithLabelValues(path, strconv.Itoa(status)).Inc()
		}
	}
}
```

That's it, this was the complete gist of the application. Now it's time to run and test if the app is registering metrics correctly.

## Running the application

Make sure you are still inside `go-prometheus-monitoring` directory in the terminal, and run the following command. Install the dependencies by running `go mod tidy` and then build and run the application by running `go run main.go`. Then visit `http://localhost:8000/health` or `http://localhost:8000/v1/users`. You should see the output `{"message": "Up and running!"}` or `{"message": "Hello from /v1/users"}`. If you are able to see this then your app is successfully up and running. 


Now, check your application's metrics by accessing the `/metrics` endpoint. 
Open `http://localhost:8000/metrics` in your browser. You should see similar output to the following.

```sh
# HELP api_http_request_error_total Total number of errors returned by the API
# TYPE api_http_request_error_total counter
api_http_request_error_total{path="/",status="404"} 1
api_http_request_error_total{path="//v1/users",status="404"} 1
api_http_request_error_total{path="/favicon.ico",status="404"} 1
# HELP api_http_request_total Total number of requests processed by the API
# TYPE api_http_request_total counter
api_http_request_total{path="/health",status="200"} 2
api_http_request_total{path="/v1/users",status="200"} 1
```

In the terminal, press `ctrl` + `c` to stop the application.

> [!Note]
> If you don't want to run the application locally, and want to run it in a Docker container, skip to next page where you create a Dockerfile and containerize the application.

## Summary

In this section, you learned how to create a Golang app to register metrics with Prometheus. By implementing middleware functions, you were able to increment the counters based on the request path and status codes.

## Next steps

In the next section, you'll learn how to containerize your application.