op-challenger: Fix metrics (#8918)

* op-challenger: Fix metrics

* op-e2e: Enable metrics when running challenger to ensure they work.

op-challenger/metrics/metrics.go

@@ -45,6 +45,9 @@ type Metricer interface {
 	DecIdleExecutors()
 }
 
+// Metrics implementation must implement RegistryMetricer to allow the metrics server to work.
+var _ opmetrics.RegistryMetricer = (*Metrics)(nil)
+
 type Metrics struct {
 	ns       string
 	registry *prometheus.Registry
@@ -70,6 +73,10 @@ type Metrics struct {
 	inflightGames prometheus.Gauge
 }
 
+func (m *Metrics) Registry() *prometheus.Registry {
+	return m.registry
+}
+
 var _ Metricer = (*Metrics)(nil)
 
 func NewMetrics() *Metrics {

op-e2e/e2eutils/challenger/helper.go

@@ -11,6 +11,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/ethereum-optimism/optimism/op-service/metrics"
 	"github.com/stretchr/testify/require"
 
 	"github.com/ethereum/go-ethereum/common"
@@ -134,6 +135,11 @@ func NewChallengerConfig(t *testing.T, l1Endpoint string, options ...Option) *co
 		// Limit concurrency to something more reasonable when there are also multiple tests executing in parallel
 		cfg.MaxConcurrency = 4
 	}
+	cfg.MetricsConfig = metrics.CLIConfig{
+		Enabled:    true,
+		ListenAddr: "127.0.0.1",
+		ListenPort: 0, // Find any available port (avoids conflicts)
+	}
 	for _, option := range options {
 		option(&cfg)
 	}