add variable item `tidb_track_aggregate_memory_usage` (#5159)
add variable item `tidb_track_aggregate_memory_usage` (#5159)

* add info

* add more info

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

* address comments

* Update system-variables.md

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>
pingcap/docs-cn,pingcap/docs-cn