feat: add prometheus metrics and dashboard

pkg/broker/metrics/server.go

@@ -0,0 +1,74 @@
+package metrics
+
+import (
+	"context"
+	_ "embed"
+	"fmt"
+	"net"
+	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+
+	"git.warky.dev/wdevs/pgsql-broker/pkg/broker/adapter"
+)
+
+//go:embed page.html
+var dashboardHTML []byte
+
+// Server is the embedded HTTP server exposing /metrics (standard Prometheus
+// exposition format) and / (a single-page HTML dashboard that polls
+// /metrics).
+type Server struct {
+	httpServer *http.Server
+	listener   net.Listener
+	logger     adapter.Logger
+}
+
+// NewServer builds a Server bound to addr (e.g. "127.0.0.1:9469"). Binding
+// happens immediately so a port conflict is reported to the caller rather
+// than surfacing later in a background goroutine.
+func NewServer(m *Metrics, addr string, logger adapter.Logger) (*Server, error) {
+	listener, err := net.Listen("tcp", addr)
+	if err != nil {
+		return nil, fmt.Errorf("failed to bind metrics server to %s: %w", addr, err)
+	}
+
+	mux := http.NewServeMux()
+	mux.Handle("/metrics", promhttp.HandlerFor(m.Registry, promhttp.HandlerOpts{}))
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/" {
+			http.NotFound(w, r)
+			return
+		}
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		_, _ = w.Write(dashboardHTML)
+	})
+
+	return &Server{
+		httpServer: &http.Server{Handler: mux},
+		listener:   listener,
+		logger:     logger.With("component", "metrics-server"),
+	}, nil
+}
+
+// Addr returns the actual bound address (useful when addr was given with a
+// ":0" port).
+func (s *Server) Addr() string {
+	return s.listener.Addr().String()
+}
+
+// Start serves in the background. It returns immediately; Serve errors
+// (other than a clean Shutdown) are logged.
+func (s *Server) Start() {
+	s.logger.Info("metrics server listening", "addr", s.Addr())
+	go func() {
+		if err := s.httpServer.Serve(s.listener); err != nil && err != http.ErrServerClosed {
+			s.logger.Error("metrics server stopped unexpectedly", "error", err)
+		}
+	}()
+}
+
+// Stop gracefully shuts down the server.
+func (s *Server) Stop(ctx context.Context) error {
+	return s.httpServer.Shutdown(ctx)
+}