feat: add legacy-api service

실험·앱 계층 설계서 3.2의 legacy-api(VM 계층). /healthz(프로세스 생존),
/v1/price/{item}(db-primary app.price 조회, DB 불가 시 503), /metrics(Prometheus).
Go 1.27.1, pgx v5.11.0, client_golang v1.24.1. 이미지는 Cloud Build로 Artifact Registry에
올린다(MIG 인스턴스가 VM SA로 pull).

legacy-api/main.go

@@ -0,0 +1,112 @@
+// legacy-api: the "VM layer" service of the self-healing lab (experiment design 3.2).
+// GET /healthz          process liveness only (MIG autohealing + ILB health check)
+// GET /v1/price/{item}  reads app.price on db-primary (503 when the DB is unreachable)
+// GET /metrics          Prometheus exposition
+package main
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"log"
+	"net/http"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+
+	_ "github.com/jackc/pgx/v5/stdlib"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+var (
+	reqTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Name: "legacy_http_requests_total", Help: "HTTP requests by route and status",
+	}, []string{"route", "status"})
+	reqDur = prometheus.NewHistogramVec(prometheus.HistogramOpts{
+		Name: "legacy_http_request_duration_seconds", Help: "HTTP request latency",
+		Buckets: []float64{.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5},
+	}, []string{"route"})
+)
+
+type statusWriter struct {
+	http.ResponseWriter
+	code int
+}
+
+func (w *statusWriter) WriteHeader(c int) { w.code = c; w.ResponseWriter.WriteHeader(c) }
+
+func instrument(route string, h http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		sw := &statusWriter{ResponseWriter: w, code: 200}
+		start := time.Now()
+		h(sw, r)
+		reqDur.WithLabelValues(route).Observe(time.Since(start).Seconds())
+		reqTotal.WithLabelValues(route, strconv.Itoa(sw.code)).Inc()
+	}
+}
+
+func writeJSON(w http.ResponseWriter, code int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(code)
+	_ = json.NewEncoder(w).Encode(v)
+}
+
+func main() {
+	prometheus.MustRegister(reqTotal, reqDur)
+	port := os.Getenv("PORT")
+	if port == "" {
+		port = "8080"
+	}
+	dsn := os.Getenv("DB_DSN")
+	var db *sql.DB
+	if dsn != "" {
+		var err error
+		if db, err = sql.Open("pgx", dsn); err != nil {
+			log.Printf("db open: %v", err)
+			db = nil
+		} else {
+			db.SetMaxOpenConns(8)
+			db.SetConnMaxLifetime(5 * time.Minute)
+		}
+	} else {
+		log.Printf("DB_DSN not set: /v1/price will answer 503")
+	}
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/healthz", instrument("healthz", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/plain")
+		_, _ = w.Write([]byte("ok\n"))
+	}))
+	mux.HandleFunc("/v1/price/", instrument("price", func(w http.ResponseWriter, r *http.Request) {
+		item := strings.TrimPrefix(r.URL.Path, "/v1/price/")
+		if item == "" || strings.Contains(item, "/") {
+			writeJSON(w, 404, map[string]string{"error": "item required"})
+			return
+		}
+		if db == nil {
+			writeJSON(w, 503, map[string]string{"error": "db unavailable"})
+			return
+		}
+		ctx, cancel := context.WithTimeout(r.Context(), 2*time.Second)
+		defer cancel()
+		var price float64
+		var updated time.Time
+		err := db.QueryRowContext(ctx, "SELECT price, updated_at FROM app.price WHERE item = $1", item).Scan(&price, &updated)
+		switch {
+		case err == sql.ErrNoRows:
+			writeJSON(w, 404, map[string]string{"error": "unknown item", "item": item})
+		case err != nil:
+			log.Printf("price query: %v", err)
+			writeJSON(w, 503, map[string]string{"error": "db unavailable"})
+		default:
+			writeJSON(w, 200, map[string]any{"item": item, "price": price, "updated_at": updated.UTC().Format(time.RFC3339)})
+		}
+	}))
+	mux.Handle("/metrics", promhttp.Handler())
+
+	srv := &http.Server{Addr: ":" + port, Handler: mux, ReadHeaderTimeout: 5 * time.Second}
+	log.Printf("legacy-api listening on :%s", port)
+	log.Fatal(srv.ListenAndServe())
+}