Added benchmark for ReferenceFilter

spec/benchmarks/lib/gitlab/markdown/reference_filter_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Markdown::ReferenceFilter, benchmark: true do
+  let(:input) do
+    html = <<-EOF
+<p>Hello @alice and @bob, how are you doing today?</p>
+<p>This is simple @dummy text to see how the @ReferenceFilter class performs
+when @processing HTML.</p>
+    EOF
+
+    Nokogiri::HTML.fragment(html)
+  end
+
+  let(:project) { create(:empty_project) }
+
+  let(:filter) { described_class.new(input, project: project) }
+
+  describe '#replace_text_nodes_matching' do
+    let(:iterations) { 6000 }
+
+    describe 'with identical input and output HTML' do
+      benchmark_subject do
+        filter.replace_text_nodes_matching(User.reference_pattern) do |content|
+          content
+        end
+      end
+
+      it { is_expected.to iterate_per_second(iterations) }
+    end
+
+    describe 'with different input and output HTML' do
+      benchmark_subject do
+        filter.replace_text_nodes_matching(User.reference_pattern) do |content|
+          '@eve'
+        end
+      end
+
+      it { is_expected.to iterate_per_second(iterations) }
+    end
+  end
+end