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

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

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+class Admin::Metrics::Measure::OpenedReportsMeasure < Admin::Metrics::Measure::BaseMeasure
+  def key
+    'opened_reports'
+  end
+
+  def total
+    Report.where(created_at: time_period).count
+  end
+
+  def previous_total
+    Report.where(created_at: previous_time_period).count
+  end
+
+  def data
+    sql = <<-SQL.squish
+      SELECT axis.*, (
+        WITH new_reports AS (
+          SELECT reports.id
+          FROM reports
+          WHERE date_trunc('day', reports.created_at)::date = axis.period
+        )
+        SELECT count(*) FROM new_reports
+      ) AS value
+      FROM (
+        SELECT generate_series(date_trunc('day', $1::timestamp)::date, date_trunc('day', $2::timestamp)::date, interval '1 day') AS period
+      ) AS axis
+    SQL
+
+    rows = ActiveRecord::Base.connection.select_all(sql, nil, [[nil, @start_at], [nil, @end_at]])
+
+    rows.map { |row| { date: row['period'], value: row['value'].to_s } }
+  end
+end