Fix errors creating project with active Prometheus service template

If the Prometheus service template were active, the service would be
inserted into the database during project creation. Later, the call to
`Projects::CreateService#after_create_actions` might attempt to NULL out
the service if the project didn't have any clusters that needed the
service, but this would fail with a foreign key violation since this is
enforced by the database constraint.

To fix this, we just assume if the template is present that it should
be there and skip the whole active check.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/212419

app/services/projects/create_service.rb

@@ -173,6 +173,10 @@ module Projects
     def create_prometheus_service
       service = @project.find_or_initialize_service(::PrometheusService.to_param)
 
+      # If the service has already been inserted in the database, that
+      # means it came from a template, and there's nothing more to do.
+      return if service.persisted?
+
       if service.prometheus_available?
         service.save!
       else

changelogs/unreleased/sh-fix-create-projects-with-prometheus-service.yml

+---
+title: Fix errors creating project with active Prometheus service template
+merge_request: 30340
+author:
+type: fixed

spec/services/projects/create_service_spec.rb

@@ -348,6 +348,7 @@ describe Projects::CreateService, '#execute' do
       project = create_project(user, opts)
 
       expect(project.services.count).to eq 1
+      expect(project.errors).to be_empty
     end
   end