Use correct k8s namespace in Prometheus queries

Before this commit the wrong namespace could have been used in
Prometheus queries for group-level installations.

app/models/clusters/platforms/kubernetes.rb

@@ -76,6 +76,10 @@ module Clusters
         end
       end
 
+      def namespace_for(project)
+        cluster.find_or_initialize_kubernetes_namespace_for_project(project)&.namespace
+      end
+
       def predefined_variables(project:)
         Gitlab::Ci::Variables::Collection.new.tap do |variables|
           variables.append(key: 'KUBE_URL', value: api_url)

lib/gitlab/prometheus/query_variables.rb

@@ -4,9 +4,13 @@ module Gitlab
   module Prometheus
     module QueryVariables
       def self.call(environment)
+        deployment_platform = environment.deployment_platform
+        namespace = deployment_platform&.namespace_for(environment.project) ||
+          deployment_platform&.actual_namespace || ''
+
         {
           ci_environment_slug: environment.slug,
-          kube_namespace: environment.deployment_platform&.actual_namespace || '',
+          kube_namespace: namespace,
           environment_filter: %{container_name!="POD",environment="#{environment.slug}"}
         }
       end

spec/lib/gitlab/prometheus/query_variables_spec.rb

@@ -4,6 +4,7 @@ require 'spec_helper'
 
 describe Gitlab::Prometheus::QueryVariables do
   describe '.call' do
+    let(:project) { environment.project }
     let(:environment) { create(:environment) }
     let(:slug) { environment.slug }
 
@@ -21,13 +22,32 @@ describe Gitlab::Prometheus::QueryVariables do
     end
 
     context 'with deployment platform' do
+      context 'with project cluster' do
         let(:kube_namespace) { environment.deployment_platform.actual_namespace }
 
         before do
-        create(:cluster, :provided_by_user, projects: [environment.project])
+          create(:cluster, :project, :provided_by_user, projects: [project])
         end
 
         it { is_expected.to include(kube_namespace: kube_namespace) }
       end
+
+      context 'with group cluster' do
+        let(:cluster) { create(:cluster, :group, :provided_by_user, groups: [group]) }
+        let(:group) { create(:group) }
+        let(:project2) { create(:project) }
+        let(:kube_namespace) { k8s_ns.namespace }
+
+        let!(:k8s_ns) { create(:cluster_kubernetes_namespace, cluster: cluster, project: project) }
+        let!(:k8s_ns2) { create(:cluster_kubernetes_namespace, cluster: cluster, project: project2) }
+
+        before do
+          group.projects << project
+          group.projects << project2
+        end
+
+        it { is_expected.to include(kube_namespace: kube_namespace) }
+      end
+    end
   end
 end