// BAD - unique per user/request
counter := promauto.NewCounterVec(
    prometheus.CounterOpts{Name: "requests_total"},
    []string{"user_id", "path"},  // millions of series!
)
counter.WithLabelValues(userID, request.URL.Path).Inc()

// GOOD - bounded label values
counter := promauto.NewCounterVec(
    prometheus.CounterOpts{Name: "requests_total"},
    []string{"method", "status_code"},  // <100 series
)
counter.WithLabelValues(r.Method, statusCode).Inc()

// BAD - using gauge for monotonic value
requestCount := promauto.NewGauge(prometheus.GaugeOpts{
    Name: "http_requests",
})
requestCount.Inc()  // should be Counter!

// GOOD
requestCount := promauto.NewCounter(prometheus.CounterOpts{
    Name: "http_requests_total",
})
requestCount.Inc()

// BAD - new metric per request
func handler(w http.ResponseWriter, r *http.Request) {
    counter := prometheus.NewCounter(...)  // creates new each time!
    prometheus.MustRegister(counter)       // panics on duplicate!
}

// GOOD - register once
var requestCounter = promauto.NewCounter(prometheus.CounterOpts{
    Name: "http_requests_total",
})

func handler(w http.ResponseWriter, r *http.Request) {
    requestCounter.Inc()
}

// BAD
duration := promauto.NewHistogram(prometheus.HistogramOpts{
    Name: "request_duration",  // no unit!
})

// GOOD
duration := promauto.NewHistogram(prometheus.HistogramOpts{
    Name: "request_duration_seconds",  // unit in name
})

var (
    httpRequests = promauto.NewCounterVec(
        prometheus.CounterOpts{
            Namespace: "myapp",
            Subsystem: "http",
            Name:      "requests_total",
            Help:      "Total HTTP requests processed",
        },
        []string{"method", "status"},
    )

    httpDuration = promauto.NewHistogramVec(
        prometheus.HistogramOpts{
            Namespace: "myapp",
            Subsystem: "http",
            Name:      "request_duration_seconds",
            Help:      "HTTP request latencies",
            Buckets:   []float64{.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10},
        },
        []string{"method"},
    )
)

func metricsMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        timer := prometheus.NewTimer(httpDuration.WithLabelValues(r.Method))
        defer timer.ObserveDuration()

        wrapped := &responseWriter{ResponseWriter: w, status: 200}
        next.ServeHTTP(wrapped, r)

        httpRequests.WithLabelValues(r.Method, strconv.Itoa(wrapped.status)).Inc()
    })
}

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

func main() {
    http.Handle("/metrics", promhttp.Handler())
    http.ListenAndServe(":9090", nil)
}