Use Time Series chart instead of area chart in panel_types

Panel types is used to build our dynamic dashboard app/assets/javascripts/monitoring/components/panel_type.vue. This panel type is still pending migration to time series chart.

Panel types is currently hidden behind the feature flag environment_metrics_additional_panel_types

Original issue:

The following discussion from !31639 (merged) should be addressed:

• @adriel started a discussion: (+1 comment)

  One final thought, I think we recently added panel_types (maybe behind a feature flag). We should probably create a follow-up merge request to add line chart in as a panel type so we can properly allow users to select it as an option.

Relates to issue https://gitlab.com/gitlab-org/gitlab-ce/issues/65412