This change adds collection of stats in key infrastructure components
of the WebSocket message broker config setup and exposes the gathered
information for logging and viewing (e.g. via JMX).
WebSocketMessageBrokerStats is a single class that assembles all
gathered information and by default logs it once every 15 minutes.
Application can also easily expose to JMX through an MBeanExporter.
A new section in the reference documentation provides a summary of
the available information.
Issue: SPR-11739
ab4864da2a