Merge pull request #466 from dSohaliya/ISSUE-437

Fix for issue 437

Author: danielqsj

kafka_exporter.go

@@ -657,22 +657,27 @@ func (e *Exporter) collect(ch chan<- prometheus.Metric) {
 						consumergroupCurrentOffset, prometheus.GaugeValue, float64(currentOffset), group.GroupId, topic, strconv.FormatInt(int64(partition), 10),
 					)
 					e.mu.Lock()
-					if offset, ok := offset[topic][partition]; ok {
-						// If the topic is consumed by that consumer group, but no offset associated with the partition
-						// forcing lag to -1 to be able to alert on that
+					currentPartitionOffset, currentPartitionOffsetError := e.client.GetOffset(topic, partition, sarama.OffsetNewest) 
+					if currentPartitionOffsetError != nil {
+						klog.Errorf("Cannot get current offset of topic %s partition %d: %v", topic, partition, currentPartitionOffsetError)
+					} else {
 						var lag int64
 						if offsetFetchResponseBlock.Offset == -1 {
 							lag = -1
 						} else {
-							lag = offset - offsetFetchResponseBlock.Offset
+							if offset, ok := offset[topic][partition]; ok {
+								if currentPartitionOffset == -1 {
+									currentPartitionOffset = offset
+								}
+							}
+							lag = currentPartitionOffset - offsetFetchResponseBlock.Offset
 							lagSum += lag
 						}
+		
 						ch <- prometheus.MustNewConstMetric(
 							consumergroupLag, prometheus.GaugeValue, float64(lag), group.GroupId, topic, strconv.FormatInt(int64(partition), 10),
 						)
-					} else {
-						klog.Errorf("No offset of topic %s partition %d, cannot get consumer group lag", topic, partition)
-					}
+					} 
 					e.mu.Unlock()
 				}
 				ch <- prometheus.MustNewConstMetric(