Stats: Rename latency to default_latency

Since it reflects only the latency of the default queue
it's more expressive like this.

lib/sidekiq/web.rb

@@ -327,7 +327,7 @@ module Sidekiq
           enqueued:   sidekiq_stats.enqueued,
           scheduled:  sidekiq_stats.scheduled_size,
           retries:    sidekiq_stats.retry_size,
-          latency:    queue.latency,
+          default_latency: queue.latency,
         },
         redis: redis_stats
       })

test/test_web.rb

@@ -326,7 +326,7 @@ class TestWeb < Sidekiq::Test
         end
 
         it 'reports latency' do
-          assert_equal 0, @response["sidekiq"]["latency"]
+          assert_equal 0, @response["sidekiq"]["default_latency"]
         end
       end