Fix nil metricsClient panic in kong binding

When no metrics backend is configured, kong's reflection-based
binding panics on nil interface values. Replace nil returns with
a noopMetrics implementation.

🤖 Generated with [Firebender](https://firebender.com)

Co-Authored-By: Firebender <[email protected]>

Author: joshfriend

cmd/gradle-cache/main.go

@@ -713,9 +713,7 @@ func main() {
 	setupLogger(cli.LogLevel)
 
 	metrics := cli.newMetricsClient()
-	if metrics != nil {
-		defer metrics.close()
-	}
+	defer metrics.close()
 
 	kctx.FatalIfErrorf(kctx.Run(ctx, metrics))
 }

cmd/gradle-cache/metrics.go

@@ -22,6 +22,14 @@ type metricsClient interface {
 	close()
 }
 
+// noopMetrics is a no-op metricsClient used when no backend is configured.
+// It exists because kong cannot bind a nil interface value.
+type noopMetrics struct{}
+
+func (noopMetrics) timing(string, int64, ...string) {}
+func (noopMetrics) gauge(string, int64, ...string)  {}
+func (noopMetrics) close()                          {}
+
 // metricsFlags are CLI flags for configuring metrics emission.
 type metricsFlags struct {
 	StatsdAddr    string   `help:"DogStatsD address (host:port) for emitting metrics. Auto-detected from DD_AGENT_HOST if not set."`
@@ -45,14 +53,14 @@ func detectStatsdAddr() string {
 
 // newMetricsClient returns a metricsClient based on the configured flags.
 // If no explicit backend is configured, auto-detects a local DD agent.
-// Returns nil if no metrics backend is available.
+// Returns a no-op client if no metrics backend is available.
 func (f *metricsFlags) newMetricsClient() metricsClient {
 	if f.StatsdAddr != "" {
 		if c := newStatsdClient(f.StatsdAddr, f.MetricsTags); c != nil {
 			return c
 		}
 		slog.Warn("failed to connect to DogStatsD, metrics disabled", "addr", f.StatsdAddr)
-		return nil
+		return noopMetrics{}
 	}
 	if f.DatadogAPIKey != "" {
 		return newDatadogAPIClient(f.DatadogAPIKey, f.MetricsTags)
@@ -64,7 +72,7 @@ func (f *metricsFlags) newMetricsClient() metricsClient {
 			return c
 		}
 	}
-	return nil
+	return noopMetrics{}
 }
 
 // ── DogStatsD (UDP) ─────────────────────────────────────────────────────────
@@ -169,16 +177,10 @@ func (d *datadogAPIClient) submit(name string, value float64, metricType string,
 
 func (d *datadogAPIClient) close() {}
 
-// emitTiming sends a timing metric if m is non-nil.
 func emitTiming(m metricsClient, name string, ms int64, tags ...string) {
-	if m != nil {
-		m.timing(name, ms, tags...)
-	}
+	m.timing(name, ms, tags...)
 }
 
-// emitGauge sends a gauge metric if m is non-nil.
 func emitGauge(m metricsClient, name string, value int64, tags ...string) {
-	if m != nil {
-		m.gauge(name, value, tags...)
-	}
+	m.gauge(name, value, tags...)
 }

cmd/gradle-cache/metrics_test.go

@@ -67,20 +67,19 @@ func TestStatsdClientGauge(t *testing.T) {
 	}
 }
 
-func TestEmitHelperNilSafe(t *testing.T) {
-	// Should not panic when metrics is nil.
-	emitTiming(nil, "test.metric", 100)
-	emitGauge(nil, "test.metric", 200)
+func TestNoopMetrics(t *testing.T) {
+	// noopMetrics should not panic.
+	var m metricsClient = noopMetrics{}
+	emitTiming(m, "test.metric", 100)
+	emitGauge(m, "test.metric", 200)
+	m.close()
 }
 
 func TestMetricsFlagsNoneConfigured(t *testing.T) {
 	// Unset DD env vars so auto-detection doesn't interfere.
 	t.Setenv("DD_AGENT_HOST", "")
 	f := &metricsFlags{}
 	m := f.newMetricsClient()
-	// May be non-nil if a local DD agent happens to be running — that's fine.
-	// Just verify it doesn't panic.
-	if m != nil {
-		m.close()
-	}
+	// Should always return a non-nil client (noopMetrics when no backend configured).
+	m.close()
 }