Polish according to Prometheus rules

[vongosling]

If possible use counters rather than gauges.

rocketmq_producer_put_size is missing units, and as a counter should end in _total.

rocketmq_group_get_latency is missing a unit and should be a Summary without quantiles if possible.

cluster and broker sound like target labels, which should come from Prometheus rather than the exporter.

Your port number isn't in the usual range from the wiki page.

More details see https://prometheus.io/docs/instrumenting/writing_exporters/