main.go

package main

import (
	"strconv"

	"github.com/gin-gonic/gin"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

// Define metrics
var (
	HttpRequestTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
		Name: "api_http_request_total",
		Help: "Total number of requests processed by the API",
	}, []string{"path", "status"})

	HttpRequestErrorTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
		Name: "api_http_request_error_total",
		Help: "Total number of errors returned by the API",
	}, []string{"path", "status"})
)

// Custom registry (without default Go metrics)
var customRegistry = prometheus.NewRegistry()

// Register metrics with custom registry
func init() {
	customRegistry.MustRegister(HttpRequestTotal, HttpRequestErrorTotal)
}

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")
}

// 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()
		}
	}
}