Document new `stacked column` in panel / metric type

We introduced a new stacked column in addition to area-chart in the monitoring Metrics / Dashboards.

This should be documented.

References

Merged MR that introduced documentation requiring review:

• gitlab-foss!31680 (closed)

Related issue(s):

Further Details

Most relevant documentation is at:

• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/project/integrations/prometheus.md#L197