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

app/lib/admin/metrics/dimension/base_dimension.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class Admin::Metrics::Dimension::BaseDimension
+  def initialize(start_at, end_at, limit)
+    @start_at = start_at&.to_datetime
+    @end_at   = end_at&.to_datetime
+    @limit    = limit&.to_i
+  end
+
+  def key
+    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
+end