Redirect to default environment when no env in params

When there is no environment in query params, redirect to the metrics
dashboard page of the default environment, rather than opaquely using
the default environment on the backend without updating the URL.

This will ensure that the query param always displays the environment
being used.

app/controllers/projects/metrics_dashboard_controller.rb

@@ -6,6 +6,8 @@ module Projects
     # app/controllers/projects/environments_controller.rb
     # See https://gitlab.com/gitlab-org/gitlab/-/issues/226002 for more details.
 
+    include Gitlab::Utils::StrongMemoize
+
     before_action :authorize_metrics_dashboard!
     before_action do
       push_frontend_feature_flag(:prometheus_computed_alerts)
@@ -15,6 +17,8 @@ module Projects
     def show
       if environment
         render 'projects/environments/metrics'
+      elsif default_environment
+        redirect_to project_metrics_dashboard_path(project, environment: default_environment)
       else
         render 'projects/environments/empty_metrics'
       end
@@ -23,10 +27,13 @@ module Projects
     private
 
     def environment
-      @environment ||=
-        if params[:environment]
-          project.environments.find(params[:environment])
-        else
+      strong_memoize(:environment) do
+        project.environments.find(params[:environment]) if params[:environment]
+      end
+    end
+
+    def default_environment
+      strong_memoize(:default_environment) do
         project.default_environment
       end
     end

spec/requests/projects/metrics_dashboard_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe 'metrics dashboard page' do
+RSpec.describe 'Projects::MetricsDashboardController' do
   let_it_be(:project) { create(:project) }
   let_it_be(:environment) { create(:environment, project: project) }
   let_it_be(:environment2) { create(:environment, project: project) }
@@ -16,12 +16,12 @@ RSpec.describe 'metrics dashboard page' do
   describe 'GET /:namespace/:project/-/metrics' do
     it 'returns 200' do
       send_request
-      expect(response).to have_gitlab_http_status(:ok)
+      expect(response).to redirect_to(dashboard_route(environment: environment))
     end
 
-    it 'assigns environment' do
+    it 'assigns default_environment' do
       send_request
-      expect(assigns(:environment).id).to eq(environment.id)
+      expect(assigns(:default_environment).id).to eq(environment.id)
     end
 
     context 'with anonymous user and public dashboard visibility' do
@@ -64,12 +64,12 @@ RSpec.describe 'metrics dashboard page' do
     let(:dashboard_path) { '.gitlab/dashboards/dashboard_path.yml' }
 
     it 'returns 200' do
-      send_request(dashboard_path: dashboard_path)
+      send_request(dashboard_path: dashboard_path, environment: environment.id)
       expect(response).to have_gitlab_http_status(:ok)
     end
 
     it 'assigns environment' do
-      send_request(dashboard_path: dashboard_path)
+      send_request(dashboard_path: dashboard_path, environment: environment.id)
       expect(assigns(:environment).id).to eq(environment.id)
     end
   end
@@ -98,14 +98,14 @@ RSpec.describe 'metrics dashboard page' do
   describe 'GET :/namespace/:project/-/metrics/:page' do
     it 'returns 200 with path param page' do
       # send_request(page: 'panel/new') cannot be used because it encodes '/'
-      get "#{dashboard_route}/panel/new"
+      get "#{dashboard_route}/panel/new?environment=#{environment.id}"
 
       expect(response).to have_gitlab_http_status(:ok)
     end
 
     it 'returns 200 with dashboard and path param page' do
       # send_request(page: 'panel/new') cannot be used because it encodes '/'
-      get "#{dashboard_route(dashboard_path: 'dashboard.yml')}/panel/new"
+      get "#{dashboard_route(dashboard_path: 'dashboard.yml')}/panel/new?environment=#{environment.id}"
 
       expect(response).to have_gitlab_http_status(:ok)
     end