How to Instrument Go Code with Prometheus Metrics

Instrument Go code for Prometheus by adding the client library, defining metrics, and exposing an HTTP endpoint for scraping.

import (
    "net/http"
    "github.com/prometheus/client_golang/prometheus"
    "github.com/prometheus/client_golang/prometheus/promhttp"
)

var requestCount = prometheus.NewCounter(
    prometheus.CounterOpts{
        Name: "http_requests_total",
        Help: "Total number of HTTP requests.",
    },
)

func init() {
    prometheus.MustRegister(requestCount)
}

func main() {
    http.HandleFunc("/metrics", promhttp.Handler().ServeHTTP)
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        requestCount.Inc()
        w.Write([]byte("Hello"))
    })
    http.ListenAndServe(":8080", nil)
}

1. Import the Prometheus client library and define your metrics using types like NewCounter or NewGauge.
2. Register your metrics with prometheus.MustRegister inside an init() function.
3. Serve the metrics by mounting promhttp.Handler() to the /metrics endpoint in your HTTP server.
4. Increment or update your metrics within your application logic whenever the relevant event occurs.

Instrumenting Go code with Prometheus metrics adds a small piece of code to your application that tracks specific events, like how many times a function runs. It then exposes a special web address that monitoring tools can visit to collect these numbers automatically. Think of it like installing a speedometer in a car so you can see how fast you are going without opening the hood.