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Golang / GoLang Production Patterns and Web Standards Interview Questions

How do you add observability (metrics and distributed tracing) to a Go service?

Production Go services expose Prometheus metrics and OpenTelemetry traces. Both integrate with Go's standard HTTP server and context-based propagation.

// Prometheus metrics with the standard prometheus/client_golang library
import (
    "github.com/prometheus/client_golang/prometheus"
    "github.com/prometheus/client_golang/prometheus/promhttp"
)

var (
    httpRequestsTotal = prometheus.NewCounterVec(
        prometheus.CounterOpts{
            Name: "http_requests_total",
            Help: "Total number of HTTP requests",
        },
        []string{"method", "path", "status"},
    )
    httpRequestDuration = prometheus.NewHistogramVec(
        prometheus.HistogramOpts{
            Name:    "http_request_duration_seconds",
            Help:    "HTTP request duration in seconds",
            Buckets: prometheus.DefBuckets,
        },
        []string{"method", "path"},
    )
)

func init() {
    prometheus.MustRegister(httpRequestsTotal, httpRequestDuration)
}

// Metrics middleware
func metricsMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        start := time.Now()
        lrw := &statusResponseWriter{ResponseWriter: w, status: 200}
        next.ServeHTTP(lrw, r)
        dur := time.Since(start).Seconds()
        path := r.URL.Path
        httpRequestsTotal.WithLabelValues(r.Method, path,
            strconv.Itoa(lrw.status)).Inc()
        httpRequestDuration.WithLabelValues(r.Method, path).Observe(dur)
    })
}

// Expose metrics endpoint
mux.Handle("GET /metrics", promhttp.Handler())

// OpenTelemetry tracing (simplified)
// otel.SetTracerProvider(tp)
// tracer := otel.Tracer("myservice")
// ctx, span := tracer.Start(ctx, "operationName")
// defer span.End()
// span.SetAttributes(attribute.String("user.id", userID))
What is the difference between a Counter and a Histogram in Prometheus?
Why should high-cardinality values (like individual user IDs) NOT be used as Prometheus label values?

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