Prevent name conflicts and rename jvm_classes_loaded

[fstab]

The ClassLoadingExports collector provides a Gauge named jvm_classes_loaded and a Counter named jvm_classes_loaded_total. This is an illegal name conflict in OpenMetrics, and this is also the root cause for prometheus/jmx_exporter#621.

This PR renames jvm_classes_loaded to jvm_classes_currently_loaded.

This is a breaking change. Prometheus queries referencing jvm_classes_loaded need to be adapted to the new name jvm_classes_currently_loaded.

Moreover, the PR adds a check in CollectorRegistry.register(Collector) to prevent collectors with such name conflicts from being registered.

The check will be performed if the Collector implements Collector.Describable, or if CollectorRegistry.autoDescribe is true (it's true by default for the CollectorRegistry.defaultRegistry and false by default for custom registries).

[fmmr]

starting solr (and other apps) with prometheus and calling DefaultExports.initialize() now fails: - #699