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

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

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class Admin::Metrics::Dimension::SourcesDimension < Admin::Metrics::Dimension::BaseDimension
+  def key
+    'sources'
+  end
+
+  def data
+    sql = <<-SQL.squish
+      SELECT oauth_applications.name, count(*) AS value
+      FROM users
+      LEFT JOIN oauth_applications ON oauth_applications.id = users.created_by_application_id
+      WHERE users.created_at BETWEEN $1 AND $2
+      GROUP BY oauth_applications.name
+      ORDER BY count(*) DESC
+      LIMIT $3
+    SQL
+
+    rows = ActiveRecord::Base.connection.select_all(sql, nil, [[nil, @start_at], [nil, @end_at], [nil, @limit]])
+
+    rows.map { |row| { key: row['name'] || 'web', human_key: row['name'] || I18n.t('admin.dashboard.website'), value: row['value'].to_s } }
+  end
+end