[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-2571] Do not attempt to write end element if the <subsystem/> has no elements
[WFLY-2571] Rework wildfly-mod_cluster-undertow module into a separate subsystem
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