0111 unregister session with timestamp

[zmstone]

Fixes 11737

Release version: v/e5.6.0

Summary

Add a new config to enable recording session unregister with a timestamp.
This timestamp can be used to count/query the number of unique clients connected during the the past X seconds/minutes/hours/days.

Demo

• Start EMQX with env EMQX_BROKER__session_history_retain='1m' to retain expired sessions for 1 minute.
• Connect a client.
• Observe session count with API:

$ curl 'http://admin:ppp@localhost:18083/api/v5/sessions_count'
1⏎

• Disconnect the client, and wait for up to 1.5min, the API should return 0

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
• Added property-based tests for code which performs user input validation
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix|breaking)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket
• 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

[ieQu1]

Duplicate of this module? https://github.com/emqx/emqx/pull/12251/files#diff-4f926b90b373b839679213fbdb3c30c784f5575bc3d4b721afce21466dded77e

[zmstone]

Duplicate of this module? https://github.com/emqx/emqx/pull/12251/files#diff-4f926b90b373b839679213fbdb3c30c784f5575bc3d4b721afce21466dded77e

sorry, I am not quite following.