add producer count metrics

[francisoliverlee]

What is the purpose of the change

add producer count metrics.

Brief changelog

XX

Verifying this change

XXXX

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[francisoliverlee]

this PR should not be merged util RocketMQ 4.9.4 release: https://github.com/apache/rocketmq/releases

[codecov-commenter]

Codecov Report

Merging #103 (5e1edb4) into master (86a6255) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##             master    #103      +/-   ##
===========================================
- Coverage      0.35%   0.33%   -0.02%     
  Complexity        2       2              
===========================================
  Files            30      31       +1     
  Lines          2270    2358      +88     
  Branches        197     207      +10     
===========================================
  Hits              8       8              
- Misses         2262    2350      +88     

Impacted Files	Coverage Δ
...rocketmq/exporter/RocketMQExporterApplication.java	0.00% <ø> (ø)
...cketmq/exporter/collector/RMQMetricsCollector.java	0.00% <0.00%> (ø)
...he/rocketmq/exporter/model/BrokerRuntimeStats.java	0.00% <0.00%> (ø)
...er/model/metrics/producer/ProducerCountMetric.java	0.00% <0.00%> (ø)
...xporter/model/metrics/producer/ProducerMetric.java	0.00% <ø> (ø)
...cketmq/exporter/service/client/MQAdminExtImpl.java	0.00% <0.00%> (ø)
...che/rocketmq/exporter/task/MetricsCollectTask.java	0.00% <0.00%> (ø)

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[ShannonDing]

4.9.4 has been released.