add logging when an alerts event can't be consumed

Kafka/Sarama won't log it (at least not by default), and this isn't
something we expect to see, so log it.

BACK-2559

Author: ewollesen

data/events/events.go

@@ -171,13 +171,15 @@ func CappedExponentialBinaryDelay(cap time.Duration) func(int) time.Duration {
 
 type AlertsEventsConsumer struct {
 	Consumer asyncevents.SaramaMessageConsumer
+	Logger   log.Logger
 }
 
 func (c *AlertsEventsConsumer) Consume(ctx context.Context,
 	session sarama.ConsumerGroupSession, message *sarama.ConsumerMessage) error {
 	err := c.Consumer.Consume(ctx, session, message)
 	if err != nil {
 		session.MarkMessage(message, fmt.Sprintf("I have given up after error: %s", err))
+		c.Logger.WithError(err).Info("Unable to consume alerts event")
 		return err
 	}
 	return nil

data/service/service/standard.go

@@ -508,6 +508,7 @@ func (s *Standard) initializeAlertsEventsHandler() error {
 		Sarama:  commonConfig.SaramaConfig,
 		MessageConsumer: &dataEvents.AlertsEventsConsumer{
 			Consumer: ec,
+			Logger:   s.Logger(),
 		},
 	}