No subscription metrics for non persistent topic consumer

[dragonls]

Describe the bug
It seems that there are no subscription throughput out metrics for non persistent topic consumer, which make it hard to know whether the consumers are working fine.

I tried to produce and consumer a non persistent topic. The metrics pulsar_subscription_msg_throughput_out and pulsar_throughput_out are always 0 for non persistent topics, but work correctly for persistent topics.

To Reproduce
Steps to reproduce the behavior:

1. Create a non persistent topic
2. Try to produce and consume the topic
3. See the metrics pulsar_subscription_msg_throughput_out and pulsar_throughput_out

Expected behavior
See the throughput out of the topic and each subscription.

Screenshots

Additional context
Pulsar version: 2.7.2

[mattisonchao]

i can work on it.

[tongsucn]

i can work on it.

@mattisonchao Hi, the problem still exists on Pulsar 2.8.2.

I think the problem locates at org.apache.pulsar.common.policies.data.stats.NonPersistentTopicStatsImpl. It owns a map object subscriptions which "overwrites" the parents' one.

In org.apache.pulsar.broker.service.nonpersistent.NonPersistentTopic.getStats, subscription information are updated to NonPersistentTopicStatsImpl.subscriptions rather than TopicStatsImpl.subscriptions.

But org.apache.pulsar.broker.stats.prometheus.NamespaceStatsAggregator.getTopicStats uses TopicStatsImpl.subscriptions to generate Prometheus metrics.

The problem also exists on publishers list and replication map in NonPersistentTopicStatsImpl. Please also pay attention to NonPersistentTopicStatsImpl.msgDropRate, which is a very important metric for non-persistent topic, you may need to add new Prometheus metric like pulsar_subscription_msg_drop_rate.