Mark CheckPrometheusHealthWorker as externally dependent

Add worker_has_external_dependencies! to health check worker. Add limit
to dev demo projects. Correct a typo.

app/workers/all_queues.yml

@@ -629,7 +629,7 @@
   :tags: []
 - :name: incident_management:clusters_applications_check_prometheus_health
   :feature_category: :incident_management
-  :has_external_dependencies: 
+  :has_external_dependencies: true
   :urgency: :low
   :resource_boundary: :unknown
   :weight: 2

app/workers/clusters/applications/check_prometheus_health_worker.rb

@@ -14,6 +14,7 @@ module Clusters
       urgency :low
 
       idempotent!
+      worker_has_external_dependencies!
 
       def perform
         demo_project_ids = Gitlab::Monitor::DemoProjects.primary_keys

lib/gitlab/monitor/demo_projects.rb

@@ -17,7 +17,7 @@ module Gitlab
         elsif ::Gitlab.staging?
           STAGING_IDS
         elsif ::Gitlab.dev_or_test_env?
-          Project.pluck(:id) # rubocop: disable CodeReuse/ActiveRecord
+          Project.limit(100).pluck(:id) # rubocop: disable CodeReuse/ActiveRecord
         else
           []
         end

spec/lib/gitlab_spec.rb

@@ -111,7 +111,7 @@ describe Gitlab do
       expect(subject).to eq true
     end
 
-    it 'is flase when not on staging' do
+    it 'is false when not on staging' do
       stub_config_setting(url: 'https://example.gitlab.com')
 
       expect(subject).to eq false