Fix consumer offset repeated calc (#269)

diff --git a/rocketmq-prometheus-exporter/src/main/java/org/apache/rocketmq/exporter/task/MetricsCollectTask.java b/rocketmq-prometheus-exporter/src/main/java/org/apache/rocketmq/exporter/task/MetricsCollectTask.java
index 504ac88..3f359e9 100644
--- a/rocketmq-prometheus-exporter/src/main/java/org/apache/rocketmq/exporter/task/MetricsCollectTask.java
+++ b/rocketmq-prometheus-exporter/src/main/java/org/apache/rocketmq/exporter/task/MetricsCollectTask.java
@@ -122,6 +122,7 @@
                         for (Map.Entry<String, Long> consumeOffsetEntry : consumeOffsetEntries) {
                             metricsService.getCollector().AddGroupOffsetMetric(clusterName,consumeOffsetEntry.getKey(), topic, group, consumeOffsetEntry.getValue());
                         }
+                        consumeOffsetMap.clear();
                     }
                 }
             }