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

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

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+class Admin::Metrics::Measure::BaseMeasure
+  def initialize(start_at, end_at)
+    @start_at = start_at&.to_datetime
+    @end_at   = end_at&.to_datetime
+  end
+
+  def key
+    raise NotImplementedError
+  end
+
+  def total
+    raise NotImplementedError
+  end
+
+  def previous_total
+    raise NotImplementedError
+  end
+
+  def data
+    raise NotImplementedError
+  end
+
+  def self.model_name
+    self.class.name
+  end
+
+  def read_attribute_for_serialization(key)
+    send(key) if respond_to?(key)
+  end
+
+  protected
+
+  def time_period
+    (@start_at...@end_at)
+  end
+
+  def previous_time_period
+    ((@start_at - length_of_period)...(@end_at - length_of_period))
+  end
+
+  def length_of_period
+    @length_of_period ||= @end_at - @start_at
+  end
+end