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

app/controllers/api/v1/admin/trends_controller.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class Api::V1::Admin::TrendsController < Api::BaseController
+  before_action :require_staff!
+  before_action :set_trends
+
+  def index
+    render json: @trends, each_serializer: REST::Admin::TagSerializer
+  end
+
+  private
+
+  def set_trends
+    @trends = TrendingTags.get(10, filtered: false)
+  end
+end