Add top-level warnings key to performance bar response

This key is useful to reduce the amount of logic needed on the frontend:
if `has_warnings` is true, then the frontend knows that the request in
question has warnings for some metric.

config/initializers/peek.rb

@@ -1,6 +1,7 @@
 require 'peek/adapters/redis'
 
 Peek::Adapters::Redis.prepend ::Gitlab::PerformanceBar::RedisAdapterWhenPeekEnabled
+Peek.singleton_class.prepend ::Gitlab::PerformanceBar::WithTopLevelWarnings
 
 Rails.application.config.peek.adapter = :redis, { client: ::Redis.new(Gitlab::Redis::Cache.params) }
 

lib/gitlab/performance_bar/with_top_level_warnings.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module PerformanceBar
+    module WithTopLevelWarnings
+      def results
+        results = super
+
+        results.merge(has_warnings: has_warnings?(results))
+      end
+
+      def has_warnings?(results)
+        results[:data].any? do |_, value|
+          value[:warnings].present?
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/performance_bar/with_top_level_warnings_spec.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require 'rspec-parameterized'
+
+describe Gitlab::PerformanceBar::WithTopLevelWarnings do
+  using RSpec::Parameterized::TableSyntax
+
+  subject { Module.new }
+
+  before do
+    subject.singleton_class.prepend(described_class)
+  end
+
+  describe '#has_warnings?' do
+    where(:has_warnings, :results) do
+      false | { data: {} }
+      false | { data: { gitaly: { warnings: [] } } }
+      true  | { data: { gitaly: { warnings: [1] } } }
+      true  | { data: { gitaly: { warnings: [] }, redis: { warnings: [1] } } }
+    end
+
+    with_them do
+      it do
+        expect(subject.has_warnings?(results)).to eq(has_warnings)
+      end
+    end
+  end
+end