fix: better protect against nil topicAttributeFunc (#618)

## ❓ Why is this being changed

systemtests are failing because of a nil topic attribute func. The topic
manager does not handle nil func.

See
https://github.com/elastic/apm-queue/actions/runs/12741942829/job/35509307227
 
## 🧑‍💻 What is being changed

set the default func in the common config instead of the metrics hook
 
## ✅ How to validate the change

run systemtests

kafka/common.go

@@ -34,6 +34,7 @@ import (
 	"github.com/twmb/franz-go/pkg/sasl/plain"
 	"github.com/twmb/franz-go/plugin/kzap"
 	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/metric"
 	"go.opentelemetry.io/otel/trace"
 	"go.uber.org/zap"
@@ -247,6 +248,12 @@ func (cfg *CommonConfig) finalize() error {
 	if cfg.TopicLogFieldFunc != nil {
 		cfg.TopicLogFieldFunc = topicFieldFunc(cfg.TopicLogFieldFunc)
 	}
+	if cfg.TopicAttributeFunc == nil {
+		cfg.TopicAttributeFunc = func(topic string) attribute.KeyValue {
+			return attribute.KeyValue{}
+		}
+	}
+
 	return errors.Join(errs...)
 }
 

kafka/common_test.go

@@ -47,6 +47,7 @@ func TestCommonConfig(t *testing.T) {
 		t.Helper()
 		err := in.finalize()
 		require.NoError(t, err)
+		in.TopicAttributeFunc = nil
 		in.TopicLogFieldFunc = nil
 		in.hooks = nil
 		assert.Equal(t, expected, in)

kafka/consumer_test.go

@@ -780,10 +780,7 @@ func TestConsumerConfigFinalizer(t *testing.T) {
 		}
 		err := cfg.finalize()
 		require.NoError(t, err)
-		assert.NotNil(t, cfg.Processor)
-		cfg.Processor = nil
-		assert.NotNil(t, cfg.Logger)
-		cfg.Logger = nil
+		assertNotNilOptions(t, &cfg)
 
 		assert.Equal(t, ConsumerConfig{
 			CommonConfig:          CommonConfig{Brokers: []string{"localhost:9092"}},
@@ -805,10 +802,7 @@ func TestConsumerConfigFinalizer(t *testing.T) {
 		}
 		err := cfg.finalize()
 		require.NoError(t, err)
-		assert.NotNil(t, cfg.Processor)
-		cfg.Processor = nil
-		assert.NotNil(t, cfg.Logger)
-		cfg.Logger = nil
+		assertNotNilOptions(t, &cfg)
 
 		assert.Equal(t, ConsumerConfig{
 			CommonConfig:          CommonConfig{Brokers: []string{"localhost:9092"}},
@@ -830,10 +824,7 @@ func TestConsumerConfigFinalizer(t *testing.T) {
 		}
 		err := cfg.finalize()
 		require.NoError(t, err)
-		assert.NotNil(t, cfg.Processor)
-		cfg.Processor = nil
-		assert.NotNil(t, cfg.Logger)
-		cfg.Logger = nil
+		assertNotNilOptions(t, &cfg)
 
 		assert.Equal(t, ConsumerConfig{
 			CommonConfig:          CommonConfig{Brokers: []string{"localhost:9092"}},
@@ -855,10 +846,7 @@ func TestConsumerConfigFinalizer(t *testing.T) {
 		}
 		err := cfg.finalize()
 		require.NoError(t, err)
-		assert.NotNil(t, cfg.Processor)
-		cfg.Processor = nil
-		assert.NotNil(t, cfg.Logger)
-		cfg.Logger = nil
+		assertNotNilOptions(t, &cfg)
 
 		assert.Equal(t, ConsumerConfig{
 			CommonConfig:          CommonConfig{Brokers: []string{"localhost:9092"}},
@@ -880,10 +868,7 @@ func TestConsumerConfigFinalizer(t *testing.T) {
 		}
 		err := cfg.finalize()
 		require.NoError(t, err)
-		assert.NotNil(t, cfg.Processor)
-		cfg.Processor = nil
-		assert.NotNil(t, cfg.Logger)
-		cfg.Logger = nil
+		assertNotNilOptions(t, &cfg)
 
 		assert.Equal(t, ConsumerConfig{
 			CommonConfig:          CommonConfig{Brokers: []string{"localhost:9092"}},
@@ -909,6 +894,17 @@ func TestConsumerConfigFinalizer(t *testing.T) {
 	})
 }
 
+func assertNotNilOptions(t testing.TB, cfg *ConsumerConfig) {
+	t.Helper()
+
+	assert.NotNil(t, cfg.Processor)
+	cfg.Processor = nil
+	assert.NotNil(t, cfg.Logger)
+	cfg.Logger = nil
+	assert.NotNil(t, cfg.TopicAttributeFunc)
+	cfg.TopicAttributeFunc = nil
+}
+
 func newConsumer(t testing.TB, cfg ConsumerConfig) *Consumer {
 	if cfg.MaxPollWait <= 0 {
 		// Lower MaxPollWait, ShutdownGracePeriod to speed up execution.

kafka/metrics.go

@@ -299,12 +299,6 @@ func newKgoHooks(mp metric.MeterProvider, namespace, topicPrefix string,
 		return nil, formatMetricError(throttlingDurationKey, err)
 	}
 
-	if topicAttributeFunc == nil {
-		topicAttributeFunc = func(topic string) attribute.KeyValue {
-			return attribute.KeyValue{}
-		}
-	}
-
 	return &metricHooks{
 		namespace:   namespace,
 		topicPrefix: topicPrefix,