adds consumer offset reset policy option to keda kafka scaler

Signed-off-by: grassiale <alessandro.grassi01@gmail.com>

pkg/scalers/kafka_scaler.go

@@ -26,10 +26,11 @@ type kafkaScaler struct {
 }
 
 type kafkaMetadata struct {
-	bootstrapServers []string
-	group            string
-	topic            string
-	lagThreshold     int64
+	bootstrapServers    []string
+	group               string
+	topic               string
+	lagThreshold        int64
+	consumerOffsetReset offsetResetPolicy
 
 	// auth
 	authMode kafkaAuthMode
@@ -42,6 +43,13 @@ type kafkaMetadata struct {
 	ca   string
 }
 
+type offsetResetPolicy string
+
+const (
+	latest   offsetResetPolicy = "latest"
+	earliest offsetResetPolicy = "earliest"
+)
+
 type kafkaAuthMode string
 
 const (
@@ -57,6 +65,7 @@ const (
 	lagThresholdMetricName   = "lagThreshold"
 	kafkaMetricType          = "External"
 	defaultKafkaLagThreshold = 10
+	defaultOffsetReset       = latest
 )
 
 var kafkaLog = logf.Log.WithName("kafka_scaler")
@@ -100,6 +109,16 @@ func parseKafkaMetadata(resolvedEnv, metadata, authParams map[string]string) (ka
 	}
 	meta.topic = metadata["topic"]
 
+	meta.consumerOffsetReset = defaultOffsetReset
+
+	if metadata["consumerOffsetReset"] != "" {
+		policy := offsetResetPolicy(metadata["offsetReset"])
+		if policy != earliest && policy != latest {
+			return meta, fmt.Errorf("err offsetReset policy %s given", policy)
+		}
+		meta.consumerOffsetReset = policy
+	}
+
 	meta.lagThreshold = defaultKafkaLagThreshold
 
 	if val, ok := metadata[lagThresholdMetricName]; ok {
@@ -295,11 +314,13 @@ func (s *kafkaScaler) getLagForPartition(partition int32, offsets *sarama.Offset
 	}
 
 	var lag int64
-	// For now, assume a consumer group that has no committed
-	// offset will read all messages from the topic. This may be
-	// something we want to allow users to configure.
+
 	if consumerOffset == sarama.OffsetNewest || consumerOffset == sarama.OffsetOldest {
-		lag = latestOffset
+		if s.metadata.consumerOffsetReset == latest {
+			lag = 0
+		} else {
+			lag = latestOffset
+		}
 	} else {
 		lag = latestOffset - consumerOffset
 	}