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

app/lib/admin/metrics/measure/active_users_measure.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+class Admin::Metrics::Measure::ActiveUsersMeasure < Admin::Metrics::Measure::BaseMeasure
+  def key
+    'active_users'
+  end
+
+  def total
+    activity_tracker.sum(time_period.first, time_period.last)
+  end
+
+  def previous_total
+    activity_tracker.sum(previous_time_period.first, previous_time_period.last)
+  end
+
+  def data
+    activity_tracker.get(time_period.first, time_period.last).map { |date, value| { date: date.to_time(:utc).iso8601, value: value.to_s } }
+  end
+
+  protected
+
+  def activity_tracker
+    @activity_tracker ||= ActivityTracker.new('activity:logins', :unique)
+  end
+
+  def time_period
+    (@start_at.to_date...@end_at.to_date)
+  end
+
+  def previous_time_period
+    ((@start_at.to_date - length_of_period)...(@end_at.to_date - length_of_period))
+  end
+end