fix DefineMetrics silently returning nil error

internal/sinks/multiwriter.go

@@ -77,7 +77,7 @@ func (mw *MultiWriter) DefineMetrics(metrics *metrics.Metrics) (err error) {
 			err = errors.Join(err, definer.DefineMetrics(metrics))
 		}
 	}
-	return nil
+	return err
 }
 
 func (mw *MultiWriter) SyncMetric(sourceName, metricName string, op SyncOp) (err error) {

internal/sinks/multiwriter_test.go

@@ -1,6 +1,7 @@
 package sinks_test
 
 import (
+	"errors"
 	"testing"
 
 	"github.com/cybertec-postgresql/pgwatch/v5/internal/metrics"
@@ -87,6 +88,42 @@ func TestWriteMeasurements(t *testing.T) {
 	assert.NoError(t, err)
 }
 
+type MockWriterWithDefiner struct {
+	shouldError bool
+}
+
+func (mw *MockWriterWithDefiner) SyncMetric(_, _ string, _ sinks.SyncOp) error {
+	return nil
+}
+
+func (mw *MockWriterWithDefiner) Write(_ metrics.MeasurementEnvelope) error {
+	return nil
+}
+
+func (mw *MockWriterWithDefiner) DefineMetrics(metrics *metrics.Metrics) error {
+	if mw.shouldError {
+		return errors.New("metric err")
+	}
+	return nil
+}
+
+func TestDefineMetrics(t *testing.T) {
+	t.Run("when there is error", func(t *testing.T) {
+		mw := &sinks.MultiWriter{}
+		mockWriter := &MockWriterWithDefiner{shouldError: true}
+		mw.AddWriter(mockWriter)
+		err := mw.DefineMetrics(&metrics.Metrics{})
+		assert.Error(t, err)
+	})
+	t.Run("when there is no error", func(t *testing.T) {
+		mw := &sinks.MultiWriter{}
+		mockWriter := &MockWriterWithDefiner{shouldError: false}
+		mw.AddWriter(mockWriter)
+		err := mw.DefineMetrics(&metrics.Metrics{})
+		assert.NoError(t, err)
+	})
+}
+
 // mockMigratableWriter implements Writer and Migrator interfaces
 type mockMigratableWriter struct {
 	migrateErr        error