Add Prometheus metrics endpoint; containerize integration tests

Prometheus metrics (internal/metrics/, cmd/maglevd/)
- New --metrics-addr flag (default :9091, env MAGLEV_METRICS_ADDR)
  serving /metrics via promhttp.
- Gauge metrics scraped on demand via a custom prometheus.Collector:
  maglev_backend_state, maglev_backend_health, maglev_backend_enabled,
  maglev_frontend_pool_backend_weight.
- Inline counter/histogram metrics updated per probe:
  maglev_probe_total (by backend, type, result, code),
  maglev_probe_duration_seconds (by backend, type),
  maglev_backend_transitions_total (by backend, from, to).
- StateSource interface in metrics package breaks the import cycle
  with checker; checker.Checker satisfies it via GetBackendInfo.

Integration tests
- Run maglevd inside a containerlab node (debian:trixie-slim with
  build/ bind-mounted) instead of on the host. Eliminates port
  collisions with any host maglevd.
- maglevc commands run via docker exec into the maglevd container.
- Add 6 Prometheus test cases: endpoint reachable, all backends
  report state=up, probe counters non-zero, duration histogram
  populated, pool weights correct, transition counters present.

cmd/maglevd/main.go

@@ -8,10 +8,13 @@ import (
 	"fmt"
 	"log/slog"
 	"net"
+	"net/http"
 	"os"
 	"os/signal"
 	"syscall"
 
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/reflection"
 
@@ -19,6 +22,7 @@ import (
 	"git.ipng.ch/ipng/vpp-maglev/internal/checker"
 	"git.ipng.ch/ipng/vpp-maglev/internal/config"
 	"git.ipng.ch/ipng/vpp-maglev/internal/grpcapi"
+	"git.ipng.ch/ipng/vpp-maglev/internal/metrics"
 )
 
 func main() {
@@ -35,6 +39,7 @@ func run() error {
 	enableReflection := flag.Bool("reflection", true, "enable gRPC server reflection (for grpcurl)")
 	configPath := stringFlag("config", "/etc/vpp-maglev/maglev.yaml", "MAGLEV_CONFIG", "path to maglev.yaml")
 	grpcAddr := stringFlag("grpc-addr", ":9090", "MAGLEV_GRPC_ADDR", "gRPC listen address")
+	metricsAddr := stringFlag("metrics-addr", ":9091", "MAGLEV_METRICS_ADDR", "Prometheus /metrics listen address (empty to disable)")
 	logLevel := stringFlag("log-level", "info", "MAGLEV_LOG_LEVEL", "log level (debug|info|warn|error)")
 	flag.Parse()
 
@@ -106,6 +111,21 @@ func run() error {
 		}
 	}()
 
+	// ---- Prometheus metrics -------------------------------------------------
+	if *metricsAddr != "" {
+		reg := prometheus.DefaultRegisterer
+		metrics.Register(reg, chkr)
+
+		mux := http.NewServeMux()
+		mux.Handle("/metrics", promhttp.Handler())
+		slog.Info("metrics-listening", "addr", *metricsAddr)
+		go func() {
+			if err := http.ListenAndServe(*metricsAddr, mux); err != nil && err != http.ErrServerClosed {
+				slog.Error("metrics-serve-error", "err", err)
+			}
+		}()
+	}
+
 	// ---- signal handling ----------------------------------------------------
 	sigCh := make(chan os.Signal, 1)
 	signal.Notify(sigCh, syscall.SIGHUP, syscall.SIGTERM, syscall.SIGINT)