Add metadata for Monitor::APM group

Mark these two API pages as belonging to the Monitor stage and
APM group.

doc/api/metrics_dashboard_annotations.md

+---
+stage: Monitor
+group: APM
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+type: concepts, howto
+---
+
 # Dashboard annotations API
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/29089) in GitLab 12.10 behind a disabled feature flag.

doc/api/metrics_user_starred_dashboards.md

+---
+stage: Monitor
+group: APM
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+type: concepts, howto
+---
+
 # User-starred metrics dashboards API
 
 The starred dashboard feature makes navigating to frequently-used dashboards easier