Simplify find_or_initialize_service precedence

Extract build_from_instance_or_template to simplify the precedence when
building the service.

app/models/project.rb

@@ -1267,22 +1267,7 @@ class Project < ApplicationRecord
   def find_or_initialize_service(name)
     return if disabled_services.include?(name)
 
-    service = find_service(services, name)
-    return service if service
-
-    instance = find_service(services_instances, name)
-
-    if instance
-      Service.build_from_integration(id, instance)
-    else
-      template = find_service(services_templates, name)
-
-      if template
-        Service.build_from_integration(id, template)
-      else
-        public_send("build_#{name}_service") # rubocop:disable GitlabSecurity/PublicSend
-      end
-    end
+    find_service(services, name) || build_from_instance_or_template(name) || public_send("build_#{name}_service") # rubocop:disable GitlabSecurity/PublicSend
   end
 
   # rubocop: disable CodeReuse/ServiceClass
@@ -2450,6 +2435,22 @@ class Project < ApplicationRecord
     services.find { |service| service.to_param == name }
   end
 
+  def build_from_instance_or_template(name)
+    instance = find_service(services_instances, name)
+    return Service.build_from_integration(id, instance) if instance
+
+    template = find_service(services_templates, name)
+    return Service.build_from_integration(id, template) if template
+  end
+
+  def services_templates
+    @services_templates ||= Service.templates
+  end
+
+  def services_instances
+    @services_instances ||= Service.instances
+  end
+
   def closest_namespace_setting(name)
     namespace.closest_setting(name)
   end
@@ -2578,14 +2579,6 @@ class Project < ApplicationRecord
     end
   end
 
-  def services_templates
-    @services_templates ||= Service.templates
-  end
-
-  def services_instances
-    @services_instances ||= Service.instances
-  end
-
   def ensure_pages_metadatum
     pages_metadatum || create_pages_metadatum!
   rescue ActiveRecord::RecordNotUnique

spec/models/project_spec.rb

@@ -5264,15 +5264,22 @@ describe Project do
       expect { subject.find_or_initialize_service('prometheus') }.not_to exceed_query_limit(control_count)
     end
 
-    it 'returns nil if service is disabled' do
+    it 'returns nil if integration is disabled' do
       allow(subject).to receive(:disabled_services).and_return(%w[prometheus])
 
       expect(subject.find_or_initialize_service('prometheus')).to be_nil
     end
 
-    it 'builds the service if instance or template does not exists' do
-      expect(subject.find_or_initialize_service('prometheus')).to be_a(PrometheusService)
-      expect(subject.find_or_initialize_service('prometheus').api_url).to be_nil
+    context 'with an existing integration' do
+      subject { create(:project) }
+
+      before do
+        create(:prometheus_service, project: subject, api_url: 'https://prometheus.project.com/')
+      end
+
+      it 'retrieves the integration' do
+        expect(subject.find_or_initialize_service('prometheus').api_url).to eq('https://prometheus.project.com/')
+      end
     end
 
     context 'with an instance-level and template integrations' do
@@ -5295,6 +5302,13 @@ describe Project do
         expect(subject.find_or_initialize_service('prometheus').api_url).to eq('https://prometheus.template.com/')
       end
     end
+
+    context 'without an exisiting integration, nor instance-level or template' do
+      it 'builds the service if instance or template does not exists' do
+        expect(subject.find_or_initialize_service('prometheus')).to be_a(PrometheusService)
+        expect(subject.find_or_initialize_service('prometheus').api_url).to be_nil
+      end
+    end
   end
 
   describe '.for_group' do