Remove depricated Monitor Metrics feature

app/assets/javascripts/alerts_settings/constants.js

app/views/clusters/clusters/_integrations.html.haml

.../shared/integrations/prometheus/_custom_metrics.html.haml

app/views/shared/integrations/prometheus/_metrics.html.haml

config/feature_flags/development/remove_monitor_metrics.yml

doc/administration/raketasks/maintenance.md

doc/development/features_inside_dot_gitlab.md

doc/development/internal_users.md

doc/operations/incident_management/alerts.md

doc/operations/incident_management/incidents.md

doc/operations/incident_management/manage_incidents.md

doc/operations/incident_management/slack.md

doc/operations/index.md

doc/operations/metrics/alerts.md

doc/operations/metrics/dashboards/default.md

doc/operations/metrics/dashboards/develop.md

...cs/dashboards/img/actions_menu_create_add_panel_v13_3.png

...ashboards/img/actions_menu_create_new_dashboard_v13_3.png

.../metrics/dashboards/img/dashboard_external_link_v13_1.png

...metrics/dashboards/img/dashboard_local_timezone_v13_1.png