Put Unleash API interface in `/api/v4`

The Unleash API is mounted in:
/api/v4/feature_flags/unleash/:project_id/

ee/lib/api/unleash.rb

@@ -2,49 +2,54 @@ module API
   class Unleash < Grape::API
     include PaginationParams
 
-    resource :feature_flags do
-      resource :projects, requirements: API::PROJECT_ENDPOINT_REQUIREMENTS do
+    namespace :feature_flags do
+      resource :unleash, requirements: API::PROJECT_ENDPOINT_REQUIREMENTS do
         params do
-          requires :id, type: String, desc: 'The ID of a project'
+          requires :project_id, type: String, desc: 'The ID of a project'
+          optional :instanceid, type: String, desc: 'The Instance ID of Unleash Client'
         end
-        route_param :id do
-          resource :unleash do
-            before do
-              authenticate_by_unleash_access_token!
-            end
-
-            get 'features' do
-              present project, with: Entities::UnleashFeatures
-            end
-
-            post 'client/register' do
-              # not supported yet
-              status :ok
-            end
-
-            post 'client/metrics' do
-              # not supported yet
-              status :ok
-            end
+        route_param :project_id do
+          before do
+            authenticate_by_unleash_access_token!
           end
-        end
-      end
 
-      helpers do
-        def project
-          @project ||= find_project(params[:id])
-        end
+          get 'features' do
+            present project, with: ::EE::API::Entities::UnleashFeatures
+          end
 
-        def unleash_instanceid
-          params[:instanceid] || env[:HTTP_UNLEASH_INSTANCEID]
-        end
+          post 'client/register' do
+            # not supported yet
+            status :ok
+          end
 
-        def authenticate_by_unleash_access_token!
-          unless Operations::FeatureFlagsAccessToken.find_by(token: unleash_instanceid, project: project)
-            unauthorized!
+          post 'client/metrics' do
+            # not supported yet
+            status :ok
           end
         end
       end
     end
+
+    helpers do
+      def project
+        @project ||= find_project(params[:project_id])
+      end
+
+      def unleash_instanceid
+        params[:instanceid] || env[:HTTP_UNLEASH_INSTANCEID]
+      end
+
+      def unleash_access_token
+        return unless unleash_instanceid
+        return unless project
+
+        @unleash_access_token ||= Operations::FeatureFlagsAccessToken.find_by(
+          token: unleash_instanceid, project: project)
+      end
+
+      def authenticate_by_unleash_access_token!
+        unauthorized! unless unleash_access_token
+      end
+    end
   end
 end

lib/api/api.rb

@@ -166,8 +166,8 @@ module API
     ## EE-specific API V4 endpoints START
     mount ::EE::API::Boards
     mount ::EE::API::GroupBoards
+    
     mount ::API::Unleash
-
     mount ::API::EpicIssues
     mount ::API::Epics
     mount ::API::Geo