Add templating key to accessors

Add templating key to accessors in PrometheusDashboard model.

app/models/performance_monitoring/prometheus_dashboard.rb

@@ -4,7 +4,7 @@ module PerformanceMonitoring
   class PrometheusDashboard
     include ActiveModel::Model
 
-    attr_accessor :dashboard, :panel_groups, :path, :environment, :priority
+    attr_accessor :dashboard, :panel_groups, :path, :environment, :priority, :templating
 
     validates :dashboard, presence: true
     validates :panel_groups, presence: true

ee/spec/models/performance_monitoring/prometheus_dashboard_spec.rb

@@ -6,6 +6,11 @@ describe PerformanceMonitoring::PrometheusDashboard do
   let(:json_content) do
     {
       "dashboard" => "Dashboard Title",
+      "templating" => {
+        "variables" => {
+          "variable1" => %w(value1 value2 value3)
+        }
+      },
       "panel_groups" => [{
         "group" => "Group Title",
         "panels" => [{