Add base url path for metrics dashboards

Base URL is needed for the metrics
dashboard for the Vue router to function.
This MR adds it on the BE and passes to FE

app/helpers/environments_helper.rb

@@ -68,6 +68,7 @@ module EnvironmentsHelper
     return {} unless environment
 
     {
+      'metrics-dashboard-base-path' => environment_metrics_path(environment),
       'current-environment-name'    => environment.name,
       'has-metrics'                 => "#{environment.has_metrics?}",
       'prometheus-status'           => "#{environment.prometheus_status}",

spec/helpers/environments_helper_spec.rb

@@ -20,6 +20,7 @@ describe EnvironmentsHelper do
       expect(metrics_data).to include(
         'settings-path' => edit_project_service_path(project, 'prometheus'),
         'clusters-path' => project_clusters_path(project),
+        'metrics-dashboard-base-path' => environment_metrics_path(environment),
         'current-environment-name' => environment.name,
         'documentation-path' => help_page_path('administration/monitoring/prometheus/index.md'),
         'empty-getting-started-svg-path' => match_asset_path('/assets/illustrations/monitoring/getting_started.svg'),