protected void loadFirstBatchTimestamp() {
        if (!rollingBasedTimestamp.isPresent()) {
            Iterator<FileChannelRecordBatch> iter = log.batches().iterator();
            if (iter.hasNext())
                rollingBasedTimestamp = OptionalLong.of(iter.next().maxTimestamp());
        }
    }
If not previously loaded,
load the timestamp of the first message into memory.
storage/src/main/java/org/apache/kafka/storage/internals/log/LogSegment.java