fix: update the will message timestamp when it is ready to publish

[lafirest]

Fixes #10680

Summary

🤖 Generated by Copilot at dcd4640

Add message tracing support to emqx_channel.erl. This module publishes messages to the broker with a timestamp field for latency measurement.

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/{ce,ee}/(feat|perf|fix)-<PR-id>.en.md files
• 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
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

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

[coveralls]

Pull Request Test Coverage Report for Build 4956612789

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 43 unchanged lines in 14 files lost coverage.
• Overall coverage decreased (-0.04%) to 81.526%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_management/src/emqx_mgmt_util.erl	1	13.79%
apps/emqx/src/emqx_broker_helper.erl	1	90.7%
apps/emqx/src/emqx_channel.erl	1	87.5%
apps/emqx/src/emqx_cm.erl	1	90.99%
apps/emqx/src/emqx_limiter/src/emqx_limiter_server.erl	1	90.21%
apps/emqx/src/emqx_session.erl	1	87.65%
apps/emqx_resource/src/emqx_resource_buffer_worker.erl	2	94.43%
apps/emqx/src/emqx_alarm.erl	2	89.92%
apps/emqx/src/emqx_banned.erl	2	89.39%
apps/emqx_gateway_exproto/src/emqx_exproto_gcli.erl	3	48.78%

Totals
Change from base Build 4954396099:	-0.04%
Covered Lines:	26231
Relevant Lines:	32175

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💛 - Coveralls

[yanzhiemq]

Bug Fixes

• Fixed the timestamp for the will message is incorrectly assigned at the session creation time, now this timestamp is the disconnected time of the session.

[yanzhiemq]

修复

• 修正了在会话创建时为遗嘱消息赋予时间戳这个错误，现在该时间戳为会话断开时间。