add variable item `tidb_track_aggregate_memory_usage` (#4508)
add variable item `tidb_track_aggregate_memory_usage` (#4508)

* add variable item `tidb_track_aggregate_memory_usage`

* Update system-variables.md

* Apply suggestions from code review

Co-authored-by: Keke Yi <4b13e11d5beb5669a80650bc9e0535372af5b679@users.noreply.github.com>

* Update system-variables.md

* Update system-variables.md

* Update system-variables.md

Co-authored-by: Keke Yi <4b13e11d5beb5669a80650bc9e0535372af5b679@users.noreply.github.com>
Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>
pingcap/docs,pingcap/docs,pingcap/docs