Add metrics to the endpoint exposed by the Prometheus plugin

There are some available metrics exposed by Jolokia (they can be found at org.apache.activemq.artemis:broker=*) that are not available when calling the REST endpoint exposed by Prometheus plugin. Include the following attributes in the output response from Prometheus plugin:

• MaxDiskUsage: Maximum limit for disk use, in percentage. Once that limit is reached any message will be blocked.
• GlobalMaxSize: Global maximum limit for in-memory messages, in bytes. It is calculated as half of the max memory available to the Java Virtual Machine, unless specified on the broker.xml configuration.
• UptimeMillis: Uptime of this server in milliseconds

Add a new metric showing the number of producers connected to an address (there is a metric showing the number of consumers per queue [ConsumerCount] but the number of producers is not available). As producers send messages to addresses, in this case the metric cannot show 'producers connected to a queue' but it could be shown at address level.

• ProducerCount: number of producers sending messages to this address