[Metricbeat] Collect client-provided name from rabbitmq connections

[kaiyan-sheng]

This feature request came from discuss forum: https://discuss.elastic.co/t/metricbeat-rabbitmq-problem/189443

User wants to know if metricbeat rabbitmq module would be able to collect the client-provided name for a collection. I was able to create a new connection in rabbitmq with a connection name and run metricbeat to collect the metrics. Turned out we don't support collecting this client-provided name right now. But this is a very simple change to make and potentially useful in my mind.

RabbitMQ 3.6.5 added the facility for the connecting client to report a friendly name string value to identify a connection for management purposes. This is strictly an identifier and, as it is client-reported, it cannot be relied upon for anything other than weak identification of connections.