Document new `line-chart` in panel / metric type

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

This should be documented.

Note: Currently part of this functionality is behind feature flags that are in process of removal.

References

Merged MR that introduced documentation requiring review:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31639

Related issue(s):

• Initial support of line charts: https://gitlab.com/gitlab-org/gitlab-ce/issues/65412
• Line charts can be used in dynamic dashboards https://gitlab.com/gitlab-org/gitlab-ce/issues/66073

Further Details

Most relevant documentation is at:

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