Fix uninitialized constant SystemDashboardService in custom dashboards

What does this MR do?

Fixes the following uninitialized constant error, when accessing a custom metrics dashboard

NameError at /root/autodevops-deploy/environments/31/metrics_dashboard.json
===========================================================================

> uninitialized constant SystemDashboardService

app/services/metrics/dashboard/custom_metric_embed_service.rb, line 43
----------------------------------------------------------------------

   38           end
   39   
   40           # All custom metrics are displayed on the system dashboard.
   41           # Nil is acceptable as we'll default to the system dashboard.
   42           def valid_dashboard?(dashboard)
>  43             dashboard.nil? || SystemDashboardService.system_dashboard?(dashboard)
   44           end
   45         end
   46   
   47         # Returns a new dashboard with only the matching
   48         # metrics from the system dashboard, stripped of

App backtrace
-------------

 - app/services/metrics/dashboard/custom_metric_embed_service.rb:43:in `valid_dashboard?'
 - app/services/metrics/dashboard/custom_metric_embed_service.rb:22:in `valid_params?'
 - lib/gitlab/metrics/dashboard/service_selector.rb:40:in `custom_metric_embed?'
 - lib/gitlab/metrics/dashboard/service_selector.rb:20:in `call'
 - ee/lib/ee/gitlab/metrics/dashboard/service_selector.rb:14:in `call'
 - lib/gitlab/metrics/dashboard/finder.rb:78:in `service_for'
 - lib/gitlab/metrics/dashboard/finder.rb:36:in `find'
 - app/controllers/concerns/metrics_dashboard.rb:9:in `metrics_dashboard'
 - ee/lib/gitlab/ip_address_state.rb:10:in `with'
 - ee/app/controllers/ee/application_controller.rb:43:in `set_current_ip_address'
 - lib/gitlab/session.rb:11:in `with_session'
 - app/controllers/application_controller.rb:446:in `set_session_storage'
 - lib/gitlab/i18n.rb:55:in `with_locale'
 - lib/gitlab/i18n.rb:61:in `with_user_locale'
 - app/controllers/application_controller.rb:440:in `set_locale'
 - lib/gitlab/middleware/rails_queue_duration.rb:27:in `call'
 - lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
 - lib/gitlab/metrics/transaction.rb:62:in `run'
 - lib/gitlab/metrics/rack_middleware.rb:17:in `call'
 - lib/gitlab/request_profiler/middleware.rb:17:in `call'
 - lib/gitlab/query_limiting/middleware.rb:17:in `block in call'
 - lib/gitlab/query_limiting/transaction.rb:39:in `run'
 - lib/gitlab/query_limiting/middleware.rb:16:in `call'
 - ee/lib/gitlab/jira/middleware.rb:19:in `call'
 - lib/gitlab/middleware/go.rb:20:in `call'
 - lib/gitlab/etag_caching/middleware.rb:13:in `call'
 - lib/gitlab/middleware/correlation_id.rb:16:in `block in call'
 - lib/gitlab/middleware/correlation_id.rb:15:in `call'
 - lib/gitlab/middleware/multipart.rb:117:in `call'
 - lib/gitlab/middleware/read_only/controller.rb:48:in `call'
 - lib/gitlab/middleware/read_only.rb:18:in `call'
 - lib/gitlab/middleware/basic_health_check.rb:25:in `call'
 - lib/gitlab/request_context.rb:32:in `call'
 - lib/gitlab/middleware/static.rb:11:in `call'
 - lib/gitlab/webpack/dev_server_middleware.rb:27:in `perform_request'
 - lib/gitlab/metrics/requests_rack_middleware.rb:49:in `call'
 - lib/gitlab/middleware/release_env.rb:12:in `call'

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• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
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• Style guides
• Database guides
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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