fix: Use separate variable to track the consume offset

This commit updates reader_service.go to use a separate variable to
track the consume offset instead of mutating the last committed
offset. I found this makes the code easier to follow as the last
committed offset does not change.

pkg/kafka/partition/reader_service.go

@@ -143,14 +143,16 @@ func (s *ReaderService) starting(ctx context.Context) error {
 
 	if lastCommittedOffset == int64(KafkaEndOffset) {
 		level.Warn(s.logger).Log("msg", fmt.Sprintf("no committed offset found, starting from %d", kafkaStartOffset))
-		lastCommittedOffset = int64(KafkaStartOffset)
+	} else {
+		level.Debug(r.logger).Log("msg", "last committed offset", "offset", lastCommittedOffset)
 	}
 
+	consumeOffset := kafkaStartOffset
 	if lastCommittedOffset >= 0 {
-		lastCommittedOffset++ // We want to begin to read from the next offset, but only if we've previously committed an offset.
+		// Read from the next offset.
+		consumeOffset = lastCommittedOffset + 1
 	}
-
-	s.reader.SetOffsetForConsumption(lastCommittedOffset)
+	s.reader.SetOffsetForConsumption(consumeOffset)
 
 	if targetLag, maxLag := s.cfg.TargetConsumerLagAtStartup, s.cfg.MaxConsumerLagAtStartup; targetLag > 0 && maxLag > 0 {
 		consumer, err := s.consumerFactory(s.committer)