Consider adding maintainers to .gitlab/CODEOWNERS file

Problem

Starting %13.0 adding groups to .gitlab/CODEOWNERS is possible which makes assigning team groups to specific code paths easy 🎉

Proposal

Make the Monitor group CODEOWNERS of code we are responsible for.

Since we don't have any guidelines on how to organize the section these options are possible:

1️⃣ Use stage name as section [Monitor]

Example:

...

[Monitor]
# APM

## Metrics
app/assets/javascripts/pages/projects/environments/metrics @gitlab-org/monitor/apm/frontend-team
app/assets/javascripts/pages/projects/prometheus/metrics @gitlab-org/monitor/apm/frontend-team
app/controllers/concerns/metrics @gitlab-org/monitor/apm/backend-team
app/finders/metrics @gitlab-org/monitor/apm/backend-team
app/models/metrics @gitlab-org/monitor/apm/backend-team
app/policies/metrics @gitlab-org/monitor/apm/backend-team
app/services/metrics @gitlab-org/monitor/apm/backend-team
app/services/projects/prometheus/metrics @gitlab-org/monitor/apm/backend-team
app/views/projects/prometheus/metrics @gitlab-org/monitor/apm/backend-team
app/workers/metrics @gitlab-org/monitor/apm/backend-team
...

# /APM

# Health

## Status Page
/ee/app/assets/javascripts/status_page_settings @gitlab-org/monitor/health/backend-team
/ee/app/finders/status_page @gitlab-org/monitor/health/backend-team
/ee/app/models/status_page @gitlab-org/monitor/health/backend-team
/ee/app/serializers/status_page @gitlab-org/monitor/health/backend-team
/ee/app/services/concerns/status_page @gitlab-org/monitor/health/backend-team
/ee/app/services/status_page @gitlab-org/monitor/health/backend-team
/ee/app/workers/status_page @gitlab-org/monitor/health/backend-team
/ee/lib/status_page @gitlab-org/monitor/health/backend-team
...

## Incident Management
...

2️⃣ Use group names as section [Monitor:APM] and [Monitor:Health] 

...

[Monitor:APM]
# Metrics
app/assets/javascripts/pages/projects/environments/metrics @gitlab-org/monitor/apm/frontend-team
app/assets/javascripts/pages/projects/prometheus/metrics @gitlab-org/monitor/apm/frontend-team
app/controllers/concerns/metrics @gitlab-org/monitor/apm/backend-team
app/finders/metrics @gitlab-org/monitor/apm/backend-team
app/models/metrics @gitlab-org/monitor/apm/backend-team
app/policies/metrics @gitlab-org/monitor/apm/backend-team
app/services/metrics @gitlab-org/monitor/apm/backend-team
app/services/projects/prometheus/metrics @gitlab-org/monitor/apm/backend-team
app/views/projects/prometheus/metrics @gitlab-org/monitor/apm/backend-team
app/workers/metrics @gitlab-org/monitor/apm/backend-team
...

[Monitor:Health]
# Status Page
/ee/app/assets/javascripts/status_page_settings @gitlab-org/monitor/health/backend-team
/ee/app/finders/status_page @gitlab-org/monitor/health/backend-team
/ee/app/models/status_page @gitlab-org/monitor/health/backend-team
/ee/app/serializers/status_page @gitlab-org/monitor/health/backend-team
/ee/app/services/concerns/status_page @gitlab-org/monitor/health/backend-team
/ee/app/services/status_page @gitlab-org/monitor/health/backend-team
/ee/app/workers/status_page @gitlab-org/monitor/health/backend-team
/ee/lib/status_page @gitlab-org/monitor/health/backend-team
...

# Incident Management
...

3️⃣ Use category names as section [Metrics], [Status Page], [Incident Management] 

[Metrics]
app/assets/javascripts/pages/projects/environments/metrics @gitlab-org/monitor/apm/frontend-team
app/assets/javascripts/pages/projects/prometheus/metrics @gitlab-org/monitor/apm/frontend-team
app/controllers/concerns/metrics @gitlab-org/monitor/apm/backend-team
app/finders/metrics @gitlab-org/monitor/apm/backend-team
app/models/metrics @gitlab-org/monitor/apm/backend-team
app/policies/metrics @gitlab-org/monitor/apm/backend-team
app/services/metrics @gitlab-org/monitor/apm/backend-team
app/services/projects/prometheus/metrics @gitlab-org/monitor/apm/backend-team
app/views/projects/prometheus/metrics @gitlab-org/monitor/apm/backend-team
app/workers/metrics @gitlab-org/monitor/apm/backend-team
...

[Status Page]
/ee/app/assets/javascripts/status_page_settings @gitlab-org/monitor/health/backend-team
/ee/app/finders/status_page @gitlab-org/monitor/health/backend-team
/ee/app/models/status_page @gitlab-org/monitor/health/backend-team
/ee/app/serializers/status_page @gitlab-org/monitor/health/backend-team
/ee/app/services/concerns/status_page @gitlab-org/monitor/health/backend-team
/ee/app/services/status_page @gitlab-org/monitor/health/backend-team
/ee/app/workers/status_page @gitlab-org/monitor/health/backend-team
/ee/lib/status_page @gitlab-org/monitor/health/backend-team
...

[Alert Management]
app/assets/javascripts/alert_management @gitlab-org/monitor/health/frontend-team
app/assets/javascripts/pages/projects/alert_management @gitlab-org/monitor/health/frontend-team
app/assets/stylesheets/pages/alert_management @gitlab-org/monitor/health/backend-team
app/finders/alert_management @gitlab-org/monitor/health/backend-team
app/graphql/mutations/alert_management @gitlab-org/monitor/health/backend-team
app/graphql/resolvers/alert_management @gitlab-org/monitor/health/backend-team
app/graphql/types/alert_management @gitlab-org/monitor/health/backend-team
app/models/alert_management @gitlab-org/monitor/health/backend-team
app/policies/alert_management @gitlab-org/monitor/health/backend-team
app/presenters/alert_management @gitlab-org/monitor/health/backend-team
app/services/alert_management @gitlab-org/monitor/health/backend-team
app/views/projects/alert_management @gitlab-org/monitor/health/backend-team
lib/gitlab/alert_management @gitlab-org/monitor/health/backend-team
...

Reorganize group structure of gitlab-org/monitor

In order to support group names like @gitlab-org/monitor/health/frontend-team or @gitlab-org/monitor/apm/backend-team etc. we'd need to tweak our current group structure of https://gitlab.com/gitlab-org/monitor (image)

🅰 gitlab-org/monitor/GROUP/END-team

We could "convert" APM and Health projects to groups and create groups like:

🅱 gitlab-org/monitor/GROUP-END-team

Alternatively, we could use @gitlab-org/monitor/health-frontend-team (note the the -) so we end up with:

Note, because of gitlab-org/gitlab#215070 (closed) we need to create a "Placeholder" project (example https://gitlab.com/gitlab-org/secure/threat-insights-frontend-team/placeholder) to make this group mentionable