feat: add :51090/metrics endpoint with Prometheus metrics listening on machine IP (#304)

Signed-off-by: Miek Gieben <miek@miek.nl>
Co-authored-by: Pasha Sviderski <me@psviderski.name>

internal/machine/metrics/server.go

@@ -0,0 +1,58 @@
+package metrics
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net"
+	"net/http"
+	"net/netip"
+	"strconv"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+const Port = 51090
+
+type Server struct {
+	*http.Server
+	listenAddr netip.Addr
+	log        *slog.Logger
+}
+
+func New(listenAddr netip.Addr) *Server {
+	mux := http.NewServeMux()
+	mux.Handle("/metrics", promhttp.Handler())
+	server := &http.Server{Handler: mux, ReadTimeout: 5 * time.Second}
+	return &Server{server, listenAddr, slog.With("component", "metrics")}
+}
+
+func (s *Server) Run(ctx context.Context) error {
+	addr := net.JoinHostPort(s.listenAddr.String(), strconv.Itoa(Port))
+	l, err := net.Listen("tcp", addr)
+	if err != nil {
+		return fmt.Errorf("listen metrics server: %w", err)
+	}
+
+	errCh := make(chan error, 1)
+
+	go func() {
+		if err := s.Serve(l); err != nil && !errors.Is(err, http.ErrServerClosed) {
+			errCh <- fmt.Errorf("metrics server failed: %w", err)
+		}
+	}()
+
+	select {
+	case err := <-errCh:
+		// The server failed on its own so there are no connections to drain.
+		return err
+	case <-ctx.Done():
+		s.log.Info("Stopping metrics server.")
+		// ctx is already cancelled so a graceful drain needs a new one with a reasonable timeout.
+		stopCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		return s.Shutdown(stopCtx)
+	}
+}