Add a "latency" stat in the dashboard

lib/sidekiq/web.rb

@@ -315,6 +315,7 @@ module Sidekiq
 
     get '/dashboard/stats' do
       sidekiq_stats = Sidekiq::Stats.new
+      queue         = Sidekiq::Queue.new
       redis_stats   = Sidekiq.redis { |conn| conn.info }.select{ |k, v| redis_keys.include? k }
 
       content_type :json
@@ -326,6 +327,7 @@ module Sidekiq
           enqueued:   sidekiq_stats.enqueued,
           scheduled:  sidekiq_stats.scheduled_size,
           retries:    sidekiq_stats.retry_size,
+          latency:    queue.latency,
         },
         redis: redis_stats
       })

test/test_web.rb

@@ -324,6 +324,10 @@ class TestWeb < Sidekiq::Test
         it 'reports scheduled' do
           assert_equal 3, @response["sidekiq"]["scheduled"]
         end
+
+        it 'reports latency' do
+          assert_equal 0, @response["sidekiq"]["latency"]
+        end
       end
 
       describe "for redis" do