Make create_environment method idempotent

Add spec to ensure that the SelfMonitoring::CreateService is idempotent.

lib/gitlab/database_importers/self_monitoring/project/create_service.rb

@@ -91,6 +91,8 @@ module Gitlab
           end
 
           def create_environment(result)
+            return success(result) if result[:project].environments.exists?
+
             environment = ::Environment.new(project_id: result[:project].id, name: 'production')
 
             if environment.save

spec/lib/gitlab/database_importers/self_monitoring/project/create_service_spec.rb

@@ -76,6 +76,14 @@ describe Gitlab::DatabaseImporters::SelfMonitoring::Project::CreateService do
 
       it_behaves_like 'has prometheus service', 'http://localhost:9090'
 
+      it 'is idempotent' do
+        result1 = subject.execute
+        expect(result1[:status]).to eq(:success)
+
+        result2 = subject.execute
+        expect(result2[:status]).to eq(:success)
+      end
+
       it "tracks successful install" do
         expect(::Gitlab::Tracking).to receive(:event).twice
         expect(::Gitlab::Tracking).to receive(:event).with('self_monitoring', 'project_created')