[Feature] Add two metrics rocketmq_active_topic_number and rocketmq_active_subscription_number

[Stephanie0002]

Is Your Feature Request Related to a Problem?

The existing observable system of this project lacks broker-side monitoring of the number of topic and subscription group.

Describe the Solution You'd Like

Add two metrics rocketmq_active_topic_number and rocketmq_active_subscription_number as following:

Type	Name	Unit	Description	Label
Gauge	rocketmq_active_topic_number	count	The number of current topics	cluster,node_type,node_id
Gauge	rocketmq_active_subscription_number	count	The number of current subscription group	cluster,node_type,node_id

Describe Alternatives You've Considered

1. Anticipated use of gauge type metric
   Gauge: A metric that can be incremented or decremented, used to describe the current state of a particular indicator, such as the remaining system memory.

2. Add metric variables to the BrokerMetricsManager class: activeTopicNum, activeSubGroupNum

3. Add corresponding constants to the BrokerMetricsConstant class

4. Build corresponding metrics inside the initStatsMetrics method of the BrokerMetricsManager class

5. Write a gauge update callback function that refreshes periodically
   a. Call this.brokerController.topicConfigManager.getTopicConfigTable().size() to retrieve the number of topics.
   b. Call brokerController.getSubscriptionGroupManager().getSubscriptionGroupTable().size() to obtain the number of subscription groups.

Additional Context

No response

[humkum]

Here you can only get the total number of topics and subscription groups, but cannot determine whether these topics and subscription groups are active. Would it be more accurate to call it rocketmq_total_topic_number & rocketmq_total_subscription_number?