refactor: emqx_statsd hot update

[zhongwencool]

Refactor emqx_statsd server

• Add statsd.tags config.
• Replace state record with map.
• Fix duplicate prefix keys.
• Improve test coverage > 90%.
• Use emqx_config_handler callback to hot update config.

PR Checklist

• Added tests for the changes
• Changed lines covered in coverage report
• Change log has been added to changes/ dir
• For internal contributor: there is a jira ticket to track this change
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• In case of non-backward compatible changes, reviewer should check this item as a write-off, and add details in Backward Compatibility section

Backward Compatibility

More information