*: update grafana-tidb-dashboard (pingcap#3067)

* *: update grafana-tidb-dashboard

* *: address comments

* Apply suggestions from code review

* Update grafana-tidb-dashboard.md

Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>
Co-authored-by: ti-srebot <66930949+ti-srebot@users.noreply.github.com>