feat(buffer_worker): log expired message count

[thalesmg]

Fixes https://emqx.atlassian.net/browse/EMQX-10165

iex(emqx@127.0.0.1)38> 2023-06-21T11:09:35.569404-03:00 [info] msg: buffer_worker_dropped_expired_messages, mfa: emqx_resource_buffer_worker:log_expired_messge_count/1, line: 982, expired_count: 900, resource_id: <<"bridge:webhook:webhook">>, worker_index: 3

Summary

🤖 Generated by Copilot at 4cc2a44

Add logging for dropped messages in emqx_resource_buffer_worker. This improves the visibility of message buffering and delivery for external resources.

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:

• [na] Added tests for the changes
• [na] Changed lines covered in coverage report
• Change log has been added to changes/{ce,ee}/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• [na] If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• [na] Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• [na] If changed package build workflow, pass this action (manual trigger)
• [na] Change log has been added to changes/ dir for user-facing artifacts update

[coveralls]

Pull Request Test Coverage Report for Build 5337220118

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• 44 unchanged lines in 16 files lost coverage.
• Overall coverage increased (+0.003%) to 81.753%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_conf/src/emqx_conf_schema.erl	1	95.0%
apps/emqx_dashboard/src/emqx_dashboard_desc_cache.erl	1	94.44%
apps/emqx_dashboard/src/emqx_dashboard_middleware.erl	1	66.67%
apps/emqx_gateway_mqttsn/src/emqx_mqttsn_frame.erl	1	64.61%
apps/emqx_resource/src/emqx_resource_buffer_worker.erl	1	93.31%
apps/emqx/src/emqx_channel.erl	1	87.71%
apps/emqx/src/emqx_message.erl	1	94.83%
apps/emqx/src/emqx_quic_data_stream.erl	1	75.21%
apps/emqx/src/emqx_session.erl	1	87.25%
apps/emqx/src/emqx_stats.erl	1	91.67%

Totals
Change from base Build 5333842210:	0.003%
Covered Lines:	29880
Relevant Lines:	36549

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💛 - Coveralls