Add inital dashboard endpoint support

app/controllers/projects/environments_controller.rb

@@ -12,6 +12,8 @@ class Projects::EnvironmentsController < Projects::ApplicationController
   before_action :expire_etag_cache, only: [:index]
   before_action only: [:metrics, :additional_metrics] do
     push_frontend_feature_flag(:metrics_time_window)
+    push_frontend_feature_flag(:environment_metrics_use_prometheus_endpoint)
+    push_frontend_feature_flag(:environment_metrics_show_multiple_dashboards)
   end
 
   def index
@@ -156,6 +158,18 @@ class Projects::EnvironmentsController < Projects::ApplicationController
     end
   end
 
+  def metrics_dashboard
+    access_denied! unless Feature.enabled?(:environment_metrics_use_prometheus_endpoint, project)
+
+    respond_to do |format|
+      format.json do
+        dashboard = MetricsDashboardService.new(@project).find(params[:dashboard])
+
+        render json: dashboard, status: :ok
+      end
+    end
+  end
+
   def search
     respond_to do |format|
       format.json do

app/services/metrics_dashboard_service.rb

+# frozen_string_literal: true
+
+# Searches a projects repository for a metrics dashboard and formats the output.
+# Expects any custom dashboards will be located in `.gitlab/dashboards`
+class MetricsDashboardService
+  DASHBOARD_ROOT = ".gitlab/dashboards"
+  DASHBOARD_EXTENSION = '.yml'
+
+  SYSTEM_DASHBOARD_NAME = 'system_dashboard'
+  SYSTEM_DASHBOARD_ROOT = "config/prometheus"
+  SYSTEM_DASHBOARD_PATH = Rails.root.join(SYSTEM_DASHBOARD_ROOT, "#{SYSTEM_DASHBOARD_NAME}#{DASHBOARD_EXTENSION}")
+
+  def initialize(project)
+    @project = project
+  end
+
+  # Returns a DB-supplemented json representation of a dashboard config file.
+  #
+  # param: dashboard_name [String] Filename of dashboard w/o an extension.
+  #     If not provided, the system dashboard will be returned.
+  def find(dashboard_name = nil)
+    unless Feature.enabled?(:environment_metrics_show_multiple_dashboards, @project)
+      return process_dashboard(system_dashboard)
+    end
+
+    dashboard = Rails.cache.fetch(cache_key(dashboard_name)) do
+      dashboard_name ? project_dashboard(dashboard) : system_dashboard
+    end
+
+    process_dashboard(dashboard)
+  end
+
+  private
+
+  # Returns the base metrics shipped with every GitLab service.
+  def system_dashboard
+    YAML.load_file(SYSTEM_DASHBOARD_PATH)
+  end
+
+  # Searches the project repo for a custom-defined dashboard.
+  def project_dashboard(dashboard_name)
+    Gitlab::Template::Finders::RepoTemplateFinder.new(
+      project,
+      DASHBOARD_ROOT,
+      DASHBOARD_EXTENSION
+    ).find(dashboard_name).read
+  end
+
+  def cache_key(dashboard_name)
+    return "metrics_dashboard_#{SYSTEM_DASHBOARD_NAME}" unless dashboard_name
+
+    "project_#{@project.id}_metrics_dashboard_#{dashboard_name}"
+  end
+
+  # TODO: "Processing" the dashboard needs to include several steps such as
+  # inserting metric ids and alert information.
+  def process_dashboard(dashboard)
+    dashboard.to_json
+  end
+end

config/routes/project.rb

@@ -218,6 +218,7 @@ constraints(::Constraints::ProjectUrlConstrainer.new) do
           get :terminal
           get :metrics
           get :additional_metrics
+          get :metrics_dashboard
           get '/terminal.ws/authorize', to: 'environments#terminal_websocket_authorize', constraints: { format: nil }
 
           get '/prometheus/api/v1/*proxy_path', to: 'environments/prometheus_api#proxy'