Add coverage for admin/metrics base classes, simplify subclass generation (#29527)

spec/lib/admin/metrics/dimension_spec.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Admin::Metrics::Dimension do
+  describe '.retrieve' do
+    subject { described_class.retrieve(reports, start_at, end_at, 5, params) }
+
+    let(:start_at) { 2.days.ago }
+    let(:end_at) { Time.now.utc }
+    let(:params) { ActionController::Parameters.new({ instance_accounts: [123], instance_languages: ['en'] }) }
+    let(:reports) { [:instance_accounts, :instance_languages] }
+
+    it 'returns instances of provided classes' do
+      expect(subject)
+        .to contain_exactly(
+          be_a(Admin::Metrics::Dimension::InstanceAccountsDimension),
+          be_a(Admin::Metrics::Dimension::InstanceLanguagesDimension)
+        )
+    end
+  end
+end