* refactor(emqx_slow_subs): refactor use moving average
* fix(emqx_slow_subs): change elapsed to latency, and fix some error
* fix(emqx_slow_subs): fix emqx_mgmt_api.erl indent
* fix(emqx_slow_subs): change api name
* fix(emqx_slow_subs): fix and improve some code
* fix(emqx_slow_subs): move clienid filed from latency_stats to session
* feat(trace): add http trace API
* feat: sub/unsub trace topic
* chore(trace): stream log use _page and _limit
* fix: elvis warning
* fix: mod_trace test failed
* fix: http api meta rename _page/_limit to _page/_limit
* fix: clientid string not working
* fix: add emqx_mod_trace to module
* fix(trace): fixed by review suggestions