Use #cluster for prometheus_adapter

We still fallback to environment.deployment_platform until we can
backfill

app/models/deployment.rb

@@ -197,7 +197,22 @@ class Deployment < ApplicationRecord
   private
 
   def prometheus_adapter
-    environment.prometheus_adapter
+    service = project.find_or_initialize_service('prometheus')
+
+    if service.can_query?
+      service
+    else
+      cluster_prometheus
+    end
+  end
+
+  # TODO remove fallback case to deployment_platform_cluster.
+  # Otherwise we will continue to pay the performance penalty described in
+  # https://gitlab.com/gitlab-org/gitlab-ce/issues/63475
+  def cluster_prometheus
+    cluster_with_fallback = cluster || deployment_platform_cluster
+
+    cluster_with_fallback.application_prometheus if cluster_with_fallback&.application_prometheus_available?
   end
 
   def ref_path

spec/factories/deployments.rb

@@ -22,6 +22,10 @@ FactoryBot.define do
       ref 'pages-deploy'
     end
 
+    trait :on_cluster do
+      cluster factory: %i(cluster provided_by_gcp)
+    end
+
     trait :running do
       status :running
     end

spec/models/deployment_spec.rb

@@ -295,6 +295,67 @@ describe Deployment do
     end
   end
 
+  describe '#has_metrics?' do
+    subject { deployment.has_metrics? }
+
+    context 'when deployment is failed' do
+      let(:deployment) { create(:deployment, :failed) }
+
+      it { is_expected.to be_falsy }
+    end
+
+    context 'when deployment is success' do
+      let(:deployment) { create(:deployment, :success) }
+
+      context 'without a monitoring service' do
+        it { is_expected.to be_falsy }
+      end
+
+      context 'with a Prometheus Service' do
+        let(:prometheus_service) { double(:prometheus_service, can_query?: true) }
+
+        before do
+          allow(deployment.project).to receive(:find_or_initialize_service).with('prometheus').and_return prometheus_service
+        end
+
+        it { is_expected.to be_truthy }
+      end
+
+      context 'with a Prometheus Service that cannot query' do
+        let(:prometheus_service) { double(:prometheus_service, can_query?: false) }
+
+        before do
+          allow(deployment.project).to receive(:find_or_initialize_service).with('prometheus').and_return prometheus_service
+        end
+
+        it { is_expected.to be_falsy }
+      end
+
+      context 'with a cluster Prometheus' do
+        let(:deployment) { create(:deployment, :success, :on_cluster) }
+        let!(:prometheus) { create(:clusters_applications_prometheus, :installed, cluster: deployment.cluster) }
+
+        before do
+          expect(deployment.cluster.application_prometheus).to receive(:can_query?).and_return(true)
+        end
+
+        it { is_expected.to be_truthy }
+      end
+
+      context 'fallback deployment platform' do
+        let(:cluster) { create(:cluster, :provided_by_user, environment_scope: '*', projects: [deployment.project]) }
+        let!(:prometheus) { create(:clusters_applications_prometheus, :installed, cluster: cluster) }
+
+        before do
+          expect(deployment.project).to receive(:deployment_platform).and_return(cluster.platform)
+          expect(cluster.application_prometheus).to receive(:can_query?).and_return(true)
+        end
+
+        it { is_expected.to be_truthy }
+      end
+    end
+  end
+
   describe '#metrics' do
     let(:deployment) { create(:deployment, :success) }
     let(:prometheus_adapter) { double('prometheus_adapter', can_query?: true) }