[WFLY-2171] Use deployment unit processors to register mod_cluster metrics
[WFLY-2090] Rework metrics to be HttpHandlers instead of ThreadSetupActions since they can run multiple times and cause ISE
[WFLY-2171] Introduce UndertowAttachments class to shelter homeless AttachmentKeys used for mod_cluster integration
[WFLY-2171] Rework running requests metric into a HttpHandler
[WFLY-2171] Only register the metrics that are actually configured to be used