Initial scaffold: Go service, Helm chart, Kustomize overlays

main.go

@@ -0,0 +1,88 @@
+package main
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"os"
+	"strconv"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+var (
+	reqTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+		Name: "http_requests_total",
+		Help: "Total number of HTTP requests.",
+	}, []string{"method", "path", "status"})
+
+	reqDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Name:    "http_request_duration_seconds",
+		Help:    "HTTP request latency in seconds.",
+		Buckets: prometheus.DefBuckets,
+	}, []string{"method", "path"})
+)
+
+func main() {
+	env := getenv("APP_ENV", "dev")
+	version := getenv("APP_VERSION", "unknown")
+	port := getenv("PORT", "8080")
+
+	logLevel := slog.LevelInfo
+	if getenv("LOG_LEVEL", "info") == "debug" {
+		logLevel = slog.LevelDebug
+	}
+	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: logLevel})))
+
+	mux := http.NewServeMux()
+	mux.Handle("/metrics", promhttp.Handler())
+	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+	mux.HandleFunc("/", instrument(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(map[string]string{
+			"env":     env,
+			"version": version,
+		})
+	}))
+
+	slog.Info("listening", "port", port, "env", env, "version", version)
+	if err := http.ListenAndServe(":"+port, mux); err != nil {
+		slog.Error("server failed", "err", err)
+		os.Exit(1)
+	}
+}
+
+func instrument(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		rw := &statusWriter{ResponseWriter: w, status: http.StatusOK}
+		start := time.Now()
+		next(rw, r)
+		dur := time.Since(start).Seconds()
+		status := strconv.Itoa(rw.status)
+		reqTotal.WithLabelValues(r.Method, r.URL.Path, status).Inc()
+		reqDuration.WithLabelValues(r.Method, r.URL.Path).Observe(dur)
+		slog.Debug("request", "method", r.Method, "path", r.URL.Path, "status", rw.status, "duration", dur)
+	}
+}
+
+type statusWriter struct {
+	http.ResponseWriter
+	status int
+}
+
+func (sw *statusWriter) WriteHeader(code int) {
+	sw.status = code
+	sw.ResponseWriter.WriteHeader(code)
+}
+
+func getenv(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}