Use Gitlab::Json to record action cable payload size

lib/gitlab/metrics/subscribers/action_cable.rb

@@ -28,7 +28,7 @@ module Gitlab
           if event.payload.present?
             channel = event.payload[:channel_class]
             operation = operation_name_from(event.payload)
-            data_size = ::ActiveSupport::JSON.encode(event.payload[:data]).bytesize
+            data_size = Gitlab::Json.generate(event.payload[:data]).bytesize
 
             transmitted_bytes_histogram.observe({ channel: channel, operation: operation }, data_size)
           end

spec/lib/gitlab/metrics/subscribers/action_cable_spec.rb

@@ -40,7 +40,7 @@ RSpec.describe Gitlab::Metrics::Subscribers::ActionCable, :request_store do
       allow(::Gitlab::Metrics).to receive(:histogram).with(
         :action_cable_transmitted_bytes, /transmit/
       ).and_return(counter)
-      message_size = ::ActiveSupport::JSON.encode(data).bytesize
+      message_size = ::Gitlab::Json.generate(data).bytesize
 
       expect(counter).to receive(:observe).with({ channel: channel_class, operation: 'event' }, message_size)