Add initial MQTT scrubber service scaffold

internal/health/server.go

@@ -0,0 +1,94 @@
+package health
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"strings"
+	"time"
+)
+
+type Server struct {
+	address string
+	server  *http.Server
+	metrics func() any
+}
+
+func NewServer(address string, metrics func() any) *Server {
+	if address == "" {
+		return nil
+	}
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/healthz", func(w http.ResponseWriter, _ *http.Request) {
+		writeJSON(w, http.StatusOK, map[string]string{"status": "ok"})
+	})
+	mux.HandleFunc("/readyz", func(w http.ResponseWriter, _ *http.Request) {
+		writeJSON(w, http.StatusOK, map[string]string{"status": "ready"})
+	})
+	mux.HandleFunc("/metrics", func(w http.ResponseWriter, _ *http.Request) {
+		if metrics == nil {
+			writeJSON(w, http.StatusOK, map[string]string{"status": "metrics_unavailable"})
+			return
+		}
+
+		w.Header().Set("Content-Type", "text/plain; version=0.0.4")
+		_, _ = w.Write([]byte(formatMetrics(metrics())))
+	})
+
+	return &Server{
+		address: address,
+		metrics: metrics,
+		server: &http.Server{
+			Addr:              address,
+			Handler:           mux,
+			ReadHeaderTimeout: 5 * time.Second,
+		},
+	}
+}
+
+func (server *Server) Start() {
+	if server == nil {
+		return
+	}
+
+	go func() {
+		slog.Info("health server started", "address", server.address)
+		if err := server.server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			slog.Error("health server stopped with error", "error", err)
+		}
+	}()
+}
+
+func (server *Server) Shutdown(ctx context.Context) error {
+	if server == nil {
+		return nil
+	}
+	return server.server.Shutdown(ctx)
+}
+
+func writeJSON(w http.ResponseWriter, status int, value any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(value)
+}
+
+func formatMetrics(snapshot any) string {
+	encoded, err := json.Marshal(snapshot)
+	if err != nil {
+		return "mqqt_scrubber_metrics_encode_error 1\n"
+	}
+
+	var fields map[string]uint64
+	if err := json.Unmarshal(encoded, &fields); err != nil {
+		return "mqqt_scrubber_metrics_decode_error 1\n"
+	}
+
+	var builder strings.Builder
+	for key, value := range fields {
+		builder.WriteString(fmt.Sprintf("mqqt_scrubber_%s %d\n", key, value))
+	}
+	return builder.String()
+}