Prometheus metrics in Go (Golang): a complete guide to monitoring

Prometheus metrics in Go (Golang): a complete guide

Prometheus is an open-source monitoring system that has become the de facto standard for collecting and analyzing metrics in modern microservice architectures. In the Go (Golang) ecosystem, the prometheus/client_golang library provides a powerful and flexible toolkit for instrumenting applications. In this article, we will thoroughly explore how to add Prometheus metrics to your Go application, configure HTTP endpoints, and create custom metric types.

Installation and setup

To get started, you need to install the official Prometheus client for Go:

go get github.com/prometheus/client_golang/prometheusgo get github.com/prometheus/client_golang/prometheus/promhttp

The first library provides the core for creating and registering metrics, the second provides a ready-made HTTP handler for the /metrics endpoint.

Basic Prometheus metric types

Prometheus supports four main metric types, each designed for specific tasks:

• Counter — a monotonically increasing value. Ideal for counting the number of requests, errors, completed tasks.
• Gauge — a value that can both increase and decrease. Used for current temperature, number of active connections, queue size.
• Histogram — collects observations (e.g., request execution time) and groups them into configurable buckets. Allows calculating quantiles.
• Summary — similar to a histogram, but calculates quantiles on the application side. Recommended for cases where quantiles are critical for business logic.

Practical example: HTTP server with metrics

Let's create a simple web application that records the number of requests, their duration, and the current number of active connections.

package main

import (    "fmt"    "net/http"    "time"

    "github.com/prometheus/client_golang/prometheus"    "github.com/prometheus/client_golang/prometheus/promhttp")

var (    // Request counter with "method" and "endpoint" labels    requestCounter = prometheus.NewCounterVec(        prometheus.CounterOpts{            Name: "http_requests_total",            Help: "Total number of HTTP requests",        },        []string{"method", "endpoint"},    )

    // Histogram for request duration    requestDuration = prometheus.NewHistogramVec(        prometheus.HistogramOpts{            Name:    "http_request_duration_seconds",            Help:    "Histogram of request duration in seconds",            Buckets: prometheus.DefBuckets, // [0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10]        },        []string{"method", "endpoint"},    )

    // Gauge for active connections    activeConnections = prometheus.NewGauge(        prometheus.GaugeOpts{            Name: "http_active_connections",            Help: "Current number of active connections",        },    ))

func init() {    // Register metrics in the Prometheus registry    prometheus.MustRegister(requestCounter)    prometheus.MustRegister(requestDuration)    prometheus.MustRegister(activeConnections)}

// middleware wraps each HTTP request to collect metricsfunc metricsMiddleware(next http.Handler) http.Handler {    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {        // Increment the active connections counter        activeConnections.Inc()        defer activeConnections.Dec()

        start := time.Now()        next.ServeHTTP(w, r)        duration := time.Since(start).Seconds()

        // Record request metrics        requestCounter.WithLabelVal