refactor: move metrics out of /rules(/:id) to /rules/:id/metrics

[sstrigler]

Fixes EMQX-7988

Summary:

• you have to query GET /rules/:id/metrics explicitly in order to get metrics to a given rule. To reset metrics of a given you resource you PUT /rules/:id/metrics/reset.
• /rules/:id/metrics/reset now returns 204 instead of 200 to comply with rest of API
• for all queries that include :id a 404 is returned in case rule does not exist

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PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Changed lines covered in coverage report
• Change log has been added to changes/ dir
• For EMQX 4.x: appup files updated (execute scripts/update-appup.sh emqx)
• 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
• Sync with dasboard team

Backward Compatibility

More information

[coveralls]

Pull Request Test Coverage Report for Build 3619333174

• 18 of 19 (94.74%) changed or added relevant lines in 2 files are covered.
• 40 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.02%) to 78.323%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/emqx_rule_engine/src/emqx_rule_engine_api.erl	12	13	92.31%

Files with Coverage Reduction	New Missed Lines	%
apps/emqx/src/emqx_broker.erl	1	84.62%
apps/emqx/src/emqx_logger.erl	1	80.49%
apps/emqx_conf/src/emqx_conf_schema.erl	1	92.31%
apps/emqx/src/emqx_connection.erl	1	88.18%
apps/emqx/src/emqx_banned.erl	2	90.91%
apps/emqx_gateway/src/bhvrs/emqx_gateway_conn.erl	15	74.11%
apps/emqx/src/emqx_reason_codes.erl	19	85.29%

Totals
Change from base Build 3619032721:	0.02%
Covered Lines:	21021
Relevant Lines:	26839

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💛 - Coveralls