Add graphs and retention metrics to admin dashboard (#16829)

app/serializers/rest/admin/measure_serializer.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class REST::Admin::MeasureSerializer < ActiveModel::Serializer
+  attributes :key, :total, :previous_total, :data
+
+  def total
+    object.total.to_s
+  end
+
+  def previous_total
+    object.previous_total.to_s
+  end
+end